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The performance merits of a mobile air conditioning (MAC) system with an orifice tube (OT) were experimentally evaluated for R1234yf relative to R134a. The effect of employing a suction/liquid line internal heat exchanger (HEX) was also investigated for R1234yf. To this end, an experimental unit employing the components of an R134a MAC system was set up, equipped with a coaxial internal HEX, and tested at various compressor speeds for both refrigerants. The air temperatures entering the condenser and evaporator were simultaneously maintained at 30, 35, and 40 °C. The R1234yf unit provided on average 22.5% lower and 6.9% higher cooling capacity than the R134a unit for the inactive and active HEX, respectively. Furthermore, the coefficient of performance (COP) for the R1234yf unit without and with HEX was on average 10.9% and 3.5% lower than that for the R134a unit, respectively. The R1234yf unit without and with HEX yielded on average 41.5% and 8.4% lower exergetic efficiency than the R134a unit, respectively. These results revealed that the use of an internal HEX in an R1234yf MAC unit with an OT compensates for the cooling capacity degradation, and the R1234yf unit has almost as high performance as the R134a unit. [ABSTRACT FROM AUTHOR]

The electrification of process industries is one of the main challenges when building a low-carbon society since they consume huge amounts of fossil fuels, generating different emissions. Heat pumps are some of the key players in the industrial sector of the carbon-neutral market. This study proposes an approach to improve the economic feasibility of heat pumps within process plants. Initial energy targeting with grand composite curves was used and supplemented with the detailed design of an evaporator and a compressor for different condensation and evaporation pressures. The trade-off between the capital cost of the heat pump and the electricity cost was investigated, and optimal configurations were selected. This case study investigates the gas fractioning unit of a polymer plant, where three heat pumps are integrated into distillation columns. The results demonstrate that the heat recovery is 174 MW and requires an additional 37.9 MW of electricity to reduce the hot utility by 212 MW. The selection of the evaporation and condensation pressures of heat pumps allows 21.5 M EUR/y to be saved for 7 years of plant operation. The emission-saving potential is estimated at 1.89 ktCO2/y. [ABSTRACT FROM AUTHOR]

Formation of frost on outdoor evaporators poses a significant challenge for air source heat pumps operating in cold climates. It is widely recognized that uneven air-flow across an evaporator can have a negative impact on both the evaporator's performance and the overall efficiency of the heat pump. This study investigates the influence of non-uniform air-flow distribution on a V-shaped outdoor evaporator as a component of an air-source transcritical CO 2 heat pump when subjected to frosting conditions. To this end, we developed and validated against independent experiments a numerical frost-formation model, and used it to study a multi-circuit evaporator of interest under different ambient conditions. It is shown that the uneven air-flow distribution results in varying frost formation patterns across different circuits of the heat exchanger, including differences in the amount of frost accumulated and its distribution among the individual tubes; the circuit with the largest frost weight accumulates 18% more frost compared to the one with the smallest in one hour under the sever frosting condition. Non-uniform frost formation also leads to air-flow distribution changing over time, and the rate at which this change occurs depends on the ambient conditions. For the presently investigated case, frost formation suppresses the non-uniformity of air-flow distribution. Moreover, evaporators with non-uniform and ideal uniform air-flows are compared and it is revealed that the difference in their capacities is more noticeable at the beginning of the frosting period. • Effect of non-uniform airflow on frosting behavior of a V-shape evaporator studied. • Different amounts of frost among circuits due to non-uniform airflow quantified. • Effect of ambient humidity on frost non-uniformity investigated. • Response of air-flow to frost formation over time investigated. • Drop in the evaporator capacity compared to the case of an ideal uniform airflow. [ABSTRACT FROM AUTHOR]

With new low-GWP HFO refrigerants, the heat transfer performances and methods for decreasing system refrigerant inventory are receiving increasing interest. In shell-and-tube heat exchangers, refrigerant distribution via pressurized liquid spray has the potential for high heat transfer performance while reducing refrigerant charge. However, no published studies have investigated LGWP refrigerants with spray evaporation on tube bundles. A test apparatus was constructed to measure the shell-side heat transfer coefficients of R1234ze(E) on bundles of tubes with two different enhanced-surface types and in two different bundle geometries at various refrigerant saturation temperatures. The results showed strong dependence on refrigerant properties, tube heat flux, enhanced-surface type, bundle geometry, and refrigerant inlet subcooling. The bundle heat transfer coefficients of R1234ze(E) were similar to that of R134a in the same test setup, usually within ±15% for similar conditions. In both cases, they first increased with the heat flux until a local maximum value was achieved. A localized dryout of the tubes at the bottom of the bundle penalized the overall bundle heat transfer coefficient at very high heat flux. For the condensing surface, bundle heat transfer coefficients rarely exceeded 10 kW/m2-K, whereas values in excess of 30 kW/m2-K were sometimes seen for the evaporating surface. [ABSTRACT FROM AUTHOR]

This research study proposes a Q-learning-based metaheuristic algorithm framework for thermal design optimization of a shell-and-tube evaporator operating with different refrigerant mixtures, which is a highly complex real-world design problem and has not been investigated yet, in previous literature approaches before. The proposed method, called QL-HEUR, uses Q-learning as a high-level heuristic to iteratively guide the competitive recently emerged low-level metaheuristic algorithms. QL-HEUR is applied to 32 unconstrained optimization benchmark functions, and results are evaluated in statistical analysis. Moreover, three multidimensional constrained optimization problems will be solved. Respective solutions unravel that QL-HEUR is very effective in finding optimum solutions to constrained and unconstrained optimization problems. QL-HEUR is employed on the design optimization of a shell-and-tube heat exchanger running with different mixture pairs as a challenging real-world benchmark case. For the design case in which R134a–R1234yf (0.8:02) mixture is considered, 8.71% of the total cost is saved compared to the preliminary design of a heat exchanger operated with pure R1234yf refrigerant. For the second design case, the application of QL-HEUR results in a decrease of 8.93% for refrigerant composition R32–R134a (0.6:0.4) in comparison with the configuration running with pure R134a. It is also seen that the heat exchanger configuration running with pure R32 refrigerant yields the lowest total cost compared to the cases accomplished by varying mixture ratios of R290 and R32. It can be concluded that the optimum configuration of the heat exchanger operated with a refrigerant mixture can be conveniently employed for minimum total cost and global warming potential. [ABSTRACT FROM AUTHOR]

• The structural model was constructed for a water evaporator operating in a low-pressure environment. • A multi-objective optimization method combining CFD simulations and the MO-SHERPA algorithm was proposed. • The offset strip fin structure and heat transfer performance of the water evaporator was optimized. The aim of this study was to investigate the optimal heat transfer performance and structure of airborne water evaporators in low-pressure environments. To achieve this, a multi-objective optimization design was performed on a plate-fin water evaporator with offset strip fins, operating in a low-pressure environment at an absolute pressure of 7 kPa. The multi-objective Simultaneous Hybrid Exploration that is Robust, Progressive and Adaptive (SHERPA) (MO-SHERPA) algorithm was used to optimize the structure of the plate–fin water evaporator under a low pressure of 7 kPa. There are three conflicting optimization objectives, including comprehensive performance factor (JF), entropy generation (S gen), and weight. The optimal Pareto frontier design points were compared and analyzed based on the three evaluation indicators. The optimization results showed that compared with the initial design point, the JF increased by 32.14% and the weight decreased by 48%, which indicated that the heat exchange performance has been significantly improved, and the weight of the heat exchanger was also reduced. The internal and external flow fields of the water evaporator, including temperature, pressure, and volume fraction of vapor, were qualitatively compared to prove the optimization effect. In addition, the influence of structural parameters on the heat transfer performance of the water evaporator were analyzed. The method proposed in this study can effectively optimize the fin structural design parameters, providing valuable guidance for the structural design of water evaporators operating in low-pressure environments. [ABSTRACT FROM AUTHOR]

In air conditioners, mini-channel heat evaporator/heat exchanger excels in performance over the traditional fin-tube counterpart. However, the uniformity of two-phase refrigerant flow distribution in mini-channel evaporators remains unclear. To enhance the flow distribution in mini-channels, a flute-type coaxial multi-opening distributor is inserted into the header of a vertical mini-channel evaporator. Using a room air conditioner (RAC) unit (cooling capacity at 7.5 kW and R410A as the refrigerant) as a sample, the opening direction and inter-opening distance along the distributor are changed to obtain the variation range where the two-phase flow distribution uniformity in the mini-channel evaporator performs the best. Considering the secondary throttle effect caused by the inserted flute-type distributor, the proper width of the preceding throttle opening is proposed based on the performance of the sample unit under the nominal condition of refrigeration. The thermal cycle pressure enthalpy diagrams of the sample unit and the infrared thermography of the mini-channel evaporator are used to verify the effect of the proposed flute-type distributor forms. The results reveal that, the most uniform refrigerant two-phase flow within the mini-channel is achieved when using a non-uniform distributor and β = 180°. It was also indicated that the CC and EER of the RAC unit are significantly increased by 2.62–6.43% and 4.04–11.39%. Additionally, the annual electricity consumption and total equivalent warming impact of the slotted distributor embedded in the microchannel evaporator were reduced by 0.40 MWh and 6.9%, respectively. [ABSTRACT FROM AUTHOR]

Micro and nanostructures can delay frost and ice buildup, improve defrosting efficiency, and reduce water retention. Here, we examine the impact of nanostructures on aluminum evaporators during frosting and defrosting inside of a 20.5 ft3 commercial food refrigerator. We use scalable manufacturing methods to generate structures on the external surfaces of the heat exchangers using bohemitization and chemical vapor deposition, rendering them superhydrophilic or superhydrophobic. Our results demonstrate a 93% reduction in water retention for the superhydrophobic heat exchanger compared to its uncoated and superhydrophilic counterparts. We conduct frosting and defrosting visualization experiments in the refrigerator to show frost pattern growth and droplet distributions on the heat exchangers. Frost was fluffier and less dense on the superhydrophobic heat exchanger compared to its counterparts which resulted in sparse droplets that are easily removed during defrosting. Furthermore, we show that the superhydrophobic heat exchanger can decrease energy consumption by 13.6% during defrosting when compared to its uncoated and superhydrophilic counterparts. We also comment on the durability of the applied coating on the heat exchangers. This study provides guidelines for the broader integration of micro and nanostructured surfaces with refrigeration and cooling appliances to create energy savings. [ABSTRACT FROM AUTHOR]

Condensers and evaporators are widely deployed as surface phase-change heat exchangers in power plants, desalination plants, and large refrigeration systems. Surface condensers and evaporators or shell and tube heat exchangers are frequently used in the process industry. However, compact condensers and evaporators such as multiport minichannel condensers and evaporators are evolving as compact, lightweight, high heat flux, and highly efficient alternatives to the bulky, space-consuming, and relatively less efficient shell and tube designs. This article reports regression analysis and new correlation development from experimental data compiled from horizontal, multiport minichannel condensers and evaporators in the past two decades. Thermo-hydraulic parameters such as the Reynolds number, Prandtl number, boiling number, Martinelli parameter, hydraulic diameter, and other variables were considered to predict the Nusselt number in such condensers and evaporators. Meta-analysis yielded statistically significant results and strongly suggested that multiport minichannels were more suitable for evaporation than condensation. [ABSTRACT FROM AUTHOR]

The topic of frost formation on the heat exchanger surface has been gaining interest since the late 1940s. Scientists and industrial engineers from many scientific and R&D units around the world have been trying to understand the nature of frosting and implement solutions to prevent such an unwanted phenomenon from having a significant impact on the performance of heat exchangers (such as a decrease in heat transfer efficiency, mechanical damage, and condensation risk). The aim of this article is to summarize the present state of knowledge dedicated to frost formation types and morphology, review, and discuss the most recent studies relevant to the challenge of frost formation, focusing on the evaporator of the domestic refrigerator. The different types of domestic refrigerators are summarized, as are the different types of evaporators inside them. Common methods of testing frost formation phenomena on the evaporator are revisited in this article, and the analysis of the most recent mathematical models is presented as well. The input and output parameters of these models are grouped, and a similar analysis is conducted for the CFD models. [ABSTRACT FROM AUTHOR]

The performance of the refrigeration system is largely influenced by the heat exchanger, and there are various methods for increasing heat exchange efficiency. In this paper, the effect of changes in the internal shape of an evaporator with different heat transfer areas on the performance under actual refrigerator operating conditions was analyzed. The type of evaporator includes a grooved evaporator and a composite grooved evaporator with a relatively large heat transfer area. The performance evaluation was evaluated by refrigerant optimization, on-off operational test, and steady state energy test. The optimum refrigerant amount for both systems is 76 g. When a composite grooved evaporator was used, monthly energy consumption was relatively saved by 2.7 kWh/month. In addition, the composite grooved evaporator has consumption and 2.2 % higher cooling capacity than the grooved evaporator. In conclusion, COP improved by 1.33 %. [ABSTRACT FROM AUTHOR]

• A quantification method is proposed to calculate the flow rate distribution of microchannel heat exchangers. • Liquid and vapor mass flow rates are calculated with the method, and it shows how refrigerant is distributed in the inlet header. • The potential of improving the heat capacity of the evaporator with uniform distribution is calculated and discussed. Maldistribution of fluid among the ports reduces the heat capacity of a microchannel heat exchanger. It is essential to understand the maldistribution mechanism to optimize the design of the heat exchanger. Quantification of the distribution helps analyze the maldistribution issue. This study proposes a method to quantify the flow distribution and capacity potential in the microchannel heat exchangers by integrating a microchannel heat exchanger model and infrared thermography. The method calculates the liquid and vapor mass flow rate at the inlet of each microchannel tube by comparing the wall temperature of a heat exchanger from simulation and the infrared image. The method is validated with tests of a 49-tube horizontal microchannel evaporator with R134a. The exit region of the top tubes has an extremely high surface temperature, which forms a superheat region. More liquid flows to the tubes located at the center of the heat exchanger. The total mass flow rate is low when the tube has a long superheat region. The heat capacity of the tested evaporator has 9.6–21.3% improvement potential when uniformly distributed. When the refrigerant outlet superheats of the evaporator is lower, the improvement potential is higher. In the superheat region, the heat flux is much lower than the average value. This quantification method provides a tool for a more profound investigation of the maldistribution issue of a microchannel evaporator, and it can be expanded to heat exchangers with different structures. [ABSTRACT FROM AUTHOR]

• Systematic investigation of liquid flooding upon compressor start-up. • Characteristic flooding curve shown for 12 combinations of heat exchangers and refrigerants. • Correlation proposed capturing influence of heat exchanger size and refrigerant properties. Refrigeration cycles with fixed speed compressors cycle on and off to modulate their time averaged cooling capacity. While the cycles are turned off, refrigerant can migrate from the condenser into the colder evaporator. The next start-up process can then pull liquid refrigerant out of the evaporator and into the compressor causing liquid flooding. This phenomenon results in a loss of latent heat for cooling, can undercool the lubricant, and may cause damage to the compression mechanism. The geometric and thermophysical conditions that lead to liquid flooding are poorly understood. Experimental data collected for four refrigerants and three heat exchangers was analyzed for flooding conditions. A correlation was fitted to the experimental data, predicting the experimentally determined charge threshold for flooding with an average deviation of 6.6% and a maximum deviation of 16.7%. Repeating the tests with other heat exchanger types and smaller or larger compressors is suggested as important future work. [ABSTRACT FROM AUTHOR]

Finned-tube heat exchangers are extensively used in industrial and daily life. As complex system designs and digital twins increase, higher demands arise for predictive modeling of finned-tube heat exchanger performance. Previous studies faced issues like disparities in evaporator performance under dry and wet conditions, inaccurate predictions of refrigerant pressure drop, and a lack of comprehensive consideration for predicting the trends in physical details. With the evolution of AI algorithms, exploring novel modeling methods becomes essential. Therefore, this study proposes a physics-informed deep residual network model for accurate performance prediction of finned-tube evaporators. The model achieved mean absolute error of 0.14 % for heat transfer rate prediction, 0.36 % for refrigerant pressure drop prediction, and 0.49 % for air pressure drop prediction. This model addresses previous issues of performance disparities under dry and wet conditions and high errors in refrigerant pressure drop prediction. The proposed deep residual network model is found much better performance in capturing physical details than other machine learning approaches. It can serve as a benchmark model for transfer learning or complex system modeling. The method of determining the structure of residual networks through a priori physical knowledge also provides a new perspective on modeling physical systems. • A ResNet model with prior physical knowledge is developed for finned-tube evaporator. • A cascade network with enhanced input improves refrigerant pressure drop prediction. • It predicts heat transfer and refrigerant pressure drop with MAPE of 0.14 % and 0.36 %. • It captures more accurate physical details than other machine learning methods. • It can serve as benchmark model for transfer learning or complex system modeling. [ABSTRACT FROM AUTHOR]

In this study, the energetic and exergetic performance of a household refrigerator with a unique compact heat exchanger composed of a mini-channel flat tube condenser and offset strip fins (OSF) is investigated experimentally by considering varying amounts of R600a (48–64 g) and capillary tube lengths (2800, 3300, and 3800 mm). The experiments have been performed for two refrigerators involving two different innovative condensers composed of mini-channel flat tube and offset strip fins which have varying orientations instead of conventional wire-on-tube condensers. The experiments were conducted in a climatic chamber at a temperature of 25 ± 0.5°C. According to the standard of IEC 62552:2015, the target temperatures of the fresh food and freezer compartments are set at 4°C and −18°C, respectively. Specific and relative exergy destructions of each component, total exergy destruction, the coefficient of performance, and second-law efficiency of the overall system with two different mini-channel condensers are reported for varying amounts of refrigerant and capillary tube lengths. The major concern of the investigation is to reveal the key components contributing to the degradation of the overall performance of the unique design. It is observed that the evaporator, which has a ratio of 52–69% in total exergy destruction within all investigated cases, is the most exergy destructive component and the exergy destruction of the evaporator decreases when the amount of R600a increases. The mini-channel condensers are the second exergy destructive components with a ratio of 19–22%. As an important outcome of this unique design’s investigation, exergy destruction of the condenser and compressor does not show a monotonic change with respect to the refrigerant amount and capillary length. Besides, the highest coefficient of performance and second-law efficiency are obtained when the amount of R600a is 48–50 g in all three capillary tube lengths. [ABSTRACT FROM AUTHOR]

In refrigeration systems, the use of natural refrigerants become widespread instead of artificial refrigerants that cause global warming. 𝑪𝑶𝟐 is a natural refrigerant used in refrigeration systems due to its environmentally friendly, low cost, ozone layer depletion potential and low global warming potential. However, since the energy efficiency of systems working with 𝑪𝑶𝟐 is lower than that of artificial refrigerants, the performance of the refrigeration system is increased by adding an internal heat exchanger. In the refrigeration industry, it is desirable in many applications to obtain environments with different evaporator temperatures. In this study, 𝑪𝑶𝟐 refrigeration cycle with two evaporators with two different evaporator temperatures is investigated separately for the cases with and without an internal heat exchanger. It is assumed that 𝑪𝑶𝟐 changes state above the critical point in the gas cooler. The dimensions of the gas cooler and evaporators are compared at different gas cooler pressures and evaporator temperatures. By obtaining the optimum gas cooler pressures, it was investigated how the situation with the internal heat exchanger improves the performance of the system and how it affects the dimensions of the system. As the pressure of the gas cooler increases, the length of the gas cooler decreases and the length of the evaporators increases. For all studied cases, the optimum gas cooler pressure is reduced by around 3% with the use of the internal heat exchanger, and the COP values are increased by around 6%. 𝑪𝑶𝑷𝒎𝒂𝒙 and optimum gas cooler pressure were obtained as 1.887 and 9685 kPa at 𝑻𝒃,𝟏 = 𝟓 °𝐂 and 𝑻𝒃,𝟐 = 𝟎 °𝐂. At this optimum gas cooler pressure, the use of the internal heat exchanger reduced the length of the gas cooler by approximately 17%, while the length of evaporator 1 increased by 6.02% and the length of evaporator 2 by 9.35%. [ABSTRACT FROM AUTHOR]

Due to a high solar energy utilizing efficiency, Direct-expansion Solar-Assisted Heat Pump (DX SAHP) system has been a research hotspot for many years, especially for the system using PV/T module as the evaporator. The heat transfer throughout the PVT evaporator is an important factor for either electrical or thermal performance. We have developed and analyzed numerically the temperature distribution of a multi-source PVT heat exchanger of serpentine shape to fit a function as evaporator/condenser for Direct-expansion Photovoltaic/Thermal-Heat Pump (DX PVTHP) system. The suitability of the developed model was based on the calculated results of the efficiency factor and simulated results of temperatures of heat exchanger and working fluid as well the working fluid pressure drop. During the numerical simulations, we have considered different temperature influencing factors such as the refrigerant and refrigerant flow rate, spacing between the adjacent tubes of the heat exchanger, backside convection and radiation effect and finned heat exchanger. Simulated results showed that the serpentine PVT heat exchanger with fins and small space between tubes (62.5 mm) at 0.02 kg/s flow rate could achieve better results in terms of uniform temperature distribution, smaller fluid pressure drop (0.078 bar) and the highest efficiency factor (0.9873) which make it recommended as the best option for suitable evaporator/condenser. The average PV temperature reached by cooled and uncooled PVT evaporator at 0.02 kg/s flow rate was 47.81 °C and 71.11 °C, respectively. It was 40.99 °C and 37.07 °C at the flow rate of 0.035 kg/s and 0.05 kg/s, respectively, and 44.44 °C for PVT evaporator with smaller space between tubes at 0.02 kg/s. The average PV temperature reached by the PVT evaporator with fins was 40.734 °C at the flow rate of 0.02 kg/s while it was 53.956 °C when the unit is insulated. [ABSTRACT FROM AUTHOR]

In this experimental study, a porous material is used inside the pipes of the evaporator as the main heat exchanging device in the air conditioning cycle. The used porous material consists of stainless steel balls of different diameters. As a case study, refrigerant R454B, which is a drop‐in replacement to refrigerant R410A, is used as a working fluid in the air conditioner thermodynamic cycle. Four different porosities were used during the experimental tests; 100% (empty tube), 46%, 40%, and 33%. This study investigated the influence of variation of porosity as well as outside air temperature and refrigerant evaporation temperature on the cycle coefficient of performance, evaporation capacity, pressure drop, and power consumption during the compression process. Measured evaporation temperatures and indoor temperatures during tests were in the range of 1.5–12°C and 18–25°C, respectively. The use of porous material in the evaporation heat exchanger resulted in a considerable increase in the cycle evaporation capacity and coefficient of performance. Varying porosity from 100% to 33% resulted in an average percent increase of cycle evaporation capacity and coefficient of performance by 48.8% and 84.3%, respectively. Also, decreasing porosity from 100% to 33% resulted in an average percent increase in power consumption during the compression process by about 27%. An average percent increase of power consumption of compressor by about 25.9% is also reported, when evaporation temperature increased from 1.5°C to 12°C. Increasing outside air temperature from 27.1°C to 39.5°C resulted in decreasing evaporation capacity and coefficient of performance by 35.2% and 34.5%, respectively, and in increasing compressor power consumption by about 14.3%. A considerable pressure drop was recorded during the evaporation process when using porous material. The volumetric evaporation capacity, as well as compressor discharge temperature, are increased by increasing evaporating temperature and by decreasing evaporator porosity. The increase in air ambient temperature resulted in a considerable increase in refrigerant mass flow rate. [ABSTRACT FROM AUTHOR]

Fin-and-tube heat exchangers can have active circuits directly adjacent to inactive circuits at part load conditions, leading to cross-fin conduction. Cross-fin conduction needs to be accounted for in heat exchanger modelling to accurately predict heat exchanger performance. This study explores how refrigerant circuitry can influence cross-fin conduction in multi-circuit evaporator coils. Our advanced segment-by-segment heat exchanger model is validated against two-phase refrigerant data obtained from our experimental facility. Three, custom designed, multi-circuit fin-and-tube heat exchanger coils with distinct circuities were used for validation purposes, including an interleaved, vertical and block circuited coils, respectively. A total of 162 experiments with several refrigerant and air inlet conditions, at full load (all circuits active) and part load (some circuits active) were carried out. At full load, for all coils, differences in coil capacity between simulated and experimental capacities were no greater than 10%, for part load the differences are larger. For the test coil with interleaved circuitry, operating with 3 of 8 circuits active, the mean average percentage error (MAPE) between experimental and simulated capacity was 2.7% when cross-fin conduction was considered, increasing to 7.7% when it was ignored. Similarly, for test coil with block circuitry, operating with 2 out of 4 circuits active, the MAPE between experimental and simulated capacity was 1.7% when cross-fin conduction was considered, and 3.4% when ignored. Results suggest that the effect of cross-fin conduction on coil capacity is directly proportional to number of inactive circuits, and it is more pronounced on interleaved circuitry compared to block circuitry. [ABSTRACT FROM AUTHOR]

The results of experimental investigations of heat transfer and a friction factor in an air channel of the minichannel heat exchanger are presented. The main aim of the analysis was to examine an influence of geometrical parameters of the fin shape with two geometries on heat transfer and flow characteristics of the air channel. The test rig was designed to monitor the parameters of the airflow during cooling by the minichannel heat exchanger. The analysis was conducted with the airflow in the range of 1-5 m/s. The temperature of the evaporation in a refrigeration system was set at 288.15 K. The energy balance of the refrigeration system was carried out. A numerical model describes the airflow through a part of the heat exchanger. Numerical simulations were validated with the experimental data. Numerical methods were used to evaluate the performance of the system and possibilities to improve the fin geometry. The characteristics of the friction factor (a measure of the pressure loss in the airflow) and the Colburn j-factor (heat transfer performance) were calculated. For the maximal velocity of the airflow, the Colburn factor was equal to 0.048 and the evaporator capacity equaled 1914 W. [ABSTRACT FROM AUTHOR]

Energy efficiency and the amount of refrigerant in heat exchangers used in cooling systems has recently been an important research subject. In this study, in order to compare and analyze different types of evaporators, industrial refrigerators were designed and in this context, test setups in accordance with TS EN ISO 23953-2 standard were produced. R290 (propane) was used as the refrigerant in the designed system. During the experiment, temperature-pressure measurements were taken at a certain point in the cooling system equipment of the products, which were cooled every minute, and test data were recorded. As a result of the data obtained from the experimental results, while the temperature difference of the air entering and leaving the evaporators was 6.797 in the first system using ½" tube diameter evaporator, it was calculated as 7.052 in the corrugated and hydrophilic coated second system using 3/8" tube diameter. In the experimental setups, the energy consumed in the first and second systems and the masses of R-290 refrigerant were measured as 24.64 kWh, 23.39 kWh and 700 grams, 430 grams, respectively. Consequently, it was calculated that the second system was 5.073% and 38.57% more efficient in terms of energy efficiency and the refrigerant mass used. Coefficient of performance values for the first and second systems were found as 2.807 and 3.013, respectively. [ABSTRACT FROM AUTHOR]

In this study, we investigated the R-1234yf distribution from a header into mini-channel tubes located in a horizontal plane, which may be of importance for cooling of an electric vehicle battery. Especially, the focus was concentrated on the effect of number of branch tubes on the two-phase distribution. The tests were conducted for the mass flux from 49 to 147 kg/m2s and inlet quality from 0.2 to 0.8. The effect of heat exchanger inclination was also investigated. The results showed that the flow distribution worsened as the number of branch tube increased. Between the normal and the parallel inlet configuration, the normal inlet yielded a better distribution, probably due to the smaller number of branch tubes. The flow distribution was mainly governed by the balance of relating forces-momentum and gravitational. Thus, a poor distribution was obtained for a tilted case, where the momentum and the gravitational forces were assisting. The data showed that a vapor quality was always beneficial to the distribution. However, the effect of mass flux was case-dependent. For the horizontal case, the mass flux pushed the flow downstream, and worsened the distribution. For the tilted case, the mass flux assisted or opposed the gravity, yielding a better distribution. [ABSTRACT FROM AUTHOR]

In this work, the effect of flow circuitry on the performance of a household freezer with a tube-on-sheet evaporator is investigated. The evaporator used in the freezer is a tube-on-sheet type where the tubes are bent and attached underneath the shelves, as in the serpentine heat exchanger. The evaporator is composed of six shelves that are located inside the cabin with a particular distance between each. Four different refrigerant circuitries are modeled to identify the one that provides a homogeneous temperature distribution while consuming less energy, and experiments are conducted by switching the refrigerants flow route between the shelves. The uniform temperature distribution at around the target temperature and the energy consumption of the entire system while achieving this target are monitored as the major performance measures of the investigation. The tests are conducted according to IEC 62552:2015, standard clause 8. The experiments found that the change of the flow circuitry has a crucial impact on the temperature distribution's uniformity and energy consumption. The type in which the refrigerant circuitry between the shelves is 2-1-6-5-4-3 has the best performance in terms of temperature distribution and energy consumption. [ABSTRACT FROM AUTHOR]

The evaporator (cooling coil or blower coil), one of the four main elements in the refrigeration cycle, was used as a finned tube heat exchanger in this study. There are difficulties in determining the parameters for sizing evaporators. It was calculated by writing a computer program due to difficulties in determining the parameters for sizing the evaporators. A program was written that has high working platform independence and makes calculations with the C# programming language of the Microsoft Visual Studio.NET platform with its structure suitable for precise roundoff error ratio mathematical operations. In the program, the thermal values of the evaporator and, accordingly, the parameters of its physical design can be calculated. With the possibilities of the thesis study, using the mathematical equations of the program, the total cooling load of the evaporator was calculated as 2.0379 kJ, the pipe surface area was 2.1070 m², the number of pipes was 19, the number of fins was 90, and when the temperature results of the manufactured evaporator were measured and compared, it was determined that the results of the program were over 92% correct. Mathematical and experimental results can be easily added thanks to the flexible programming structure in order to obtain more efficient results in future versions of this thesis study prepared as a starter program. [ABSTRACT FROM AUTHOR]

The heat exchangers are essential components in the CO 2 heat pump-driven liquid desiccant dehumidification system since the heat transfer performance of heat exchangers directly affects the energy efficiency of the system. However, the heat transfer characteristics of CO 2 and liquid desiccant in the heat exchanger have yet to be fully revealed, resulting in a lack of appropriate strategies to improve the heat transfer performance. To address this issue, numerical gas cooler and evaporator models are developed and verified. The effects of parameters such as the weight concentration of the CaCl 2 aqueous solution, the CO 2 inlet pressure, mass flow rates, and the length of the heat exchanger on the heat transfer performance are investigated respectively under various operating conditions. Results show that the heat exchange load of the gas cooler can be significantly impacted by various inlet parameters of both fluids. In particular, the increase in the weight concentration from 36 to 54 wt.% deteriorates the heat exchange load by over 15 %. Nevertheless, the heat exchange load of the evaporator is hardly influenced by the weight concentration. This is attributed to a great gap in the two fluids' heat capacity rates, causing the evaporator to be in the maximum heat transfer state. The sign of the maximum heat transfer state of the heat exchanger is whether the thermal effectiveness is close to 1, which is related to the heat transfer area. Therefore, modifying the heat exchanger length could avoid the thermal effectiveness approaching 1. Results indicate that the appropriate length of the evaporator is 3–4 m under the designed operating conditions. • Heat transfer process for CO 2 and CaCl 2 aqueous solution is simulated accurately. • The heat transfer performance in the gas cooler and evaporator is investigated. • The evaporator is in the maximum heat transfer state in the designed condition. • The appropriate lengths of the gas cooler and evaporator are 14 m and 3–4 m. [ABSTRACT FROM AUTHOR]

A mathematical model is proposed for predicting the air-side pressure drop and heat-transfer rate of straight fin-tube heat exchangers of a household refrigerator under nofrosting conditions. The six-sigma method is employed to optimize the preliminary critical-to-quality (CTQ). The proposed model is consistent with the experimental data with an error of less than 15 %. The numerical results agree well with the experimental data. It can be applied to evaluate the thermal-hydraulic performance of a fin-tube heat exchanger with varying fin pitch along the airflow direction. [ABSTRACT FROM AUTHOR]

Part I showed that Carbon Dioxide (CO 2) is a promising candidate for use in vertical direct expansion ground evaporators, as it requires smaller pipe diameters within the geothermal heat exchanger and lower power for refrigerant circulation compared to conventional refrigerants. Furthermore, pressure change along the geothermal evaporator influences marginally the evaporation temperature leading to quasi-isothermal evaporation of CO 2. However, in single U-pipe configurations, small pipe diameter means lower pipe surface area and therefore higher thermal resistance. In Part II, all these advantages are taken into consideration toward proposing new geothermal evaporator configurations with multiple U-pipes (2 to 4 U-pipes in one borehole). R410A is also compared against CO 2 for the new configurations. Operating parameters of all configurations were chosen so that CO 2 and R410A are both subjected to a complete evaporation for equal total pipe surface area and equal heat extraction rate. Although all configurations have same total pipe surface area, it is shown that adding U-pipes and reducing pipe diameters diminishes borehole thermal resistance significantly. A novel configuration is also proposed to lower pressure drop of CO 2 in the 4-U borehole configuration by insulating a portion of the downward U-pipe legs. Decreasing pipe dimensions by adding U-pipes while keeping total pipe surface area constant offers an opportunity to bundle pipes closer to each other and make the evaporator more compact as seen in the proposed Polymer Block configuration. [ABSTRACT FROM AUTHOR]

CO 2 is a promising candidate for replacing synthetic refrigerants used in direct-expansion ground-source heat pumps (DX-GSHPs). The present paper, as the first part of two, presents a numerical performance comparison of a vertical single U-pipe direct expansion geothermal evaporator using different refrigerants; namely R744, R410A, R22, R407C, R12345yf, and R134a. Each refrigerant was tested through a borehole heat exchanger (BHE) performance simulation at a constant rate of heat extraction. Operating parameters were chosen so that each refrigerant is subjected to a complete evaporation cycle, and a similar pressure drop from the inlet to outlet of the borehole. Several BHE parameters such as fluid temperature, pressure, mass flow rate, required pipe dimensions, and required power for fluid circulation were evaluated for different refrigerants. Finally, the overall performance of each refrigerant was characterized by the heat exchange capacity for a unit of mass flow rate and pipe surface area. Results showed that CO 2 offers unique characteristics favoring the design of compact BHEs, as it required the smallest mass flow and pipe surface area. It is also concluded that the performance of a BHE with CO 2 can be significantly improved through using unconventional configurations. This will be discussed further in the second part (PART II). [ABSTRACT FROM AUTHOR]

In this study, implementation of a thermosyphon heat exchanger into a Stirling refrigeration system is demonstrated by means of both theoretical and experimental analyses. R-134a was employed as the working fluid which formed a liquid column at the condenser exit, enabling a two-phase flow through the evaporator. Experiments were conducted in a controlled chamber. Cooling capacity, coefficient of performance, and two-phase pressure drop of the cooler were compared at two different voltages. The flow regime for the refrigerant in the evaporator was found to be stratified based on the calculated wavy flow and Lockhart–Martinelli parameters. [ABSTRACT FROM AUTHOR]

Desiccant coated heat exchangers (DCHEs), utilizing an inner cooling source to remove sorption heat, are promising alternatives for evaporators and condensers (DCEs/DCCs) in vapor compression (VC) heat pumps. A mathematical model is necessary to facilitate the design, analysis and performance prediction of the component and the relevant systems. In this study, a three-dimensional model of DCEs/DCCs is proposed, accounting for the two-phase phenomena, the periodical switchover, the solid-side resistance, the fluid transport in multiple directions and the coupled heat and mass transfer. Study reveals that, high latent load (inlet humidity) reduces the sensible heat handling capacity of the DCEs, while the dehumidification capacity is almost independent of its sensible heat load. DCEs possess a satisfying effect of dehumidification above the dew point, thus it is unnecessary to employ low evaporation temperature. Meanwhile, the adsorption-desorption temperature difference of 30°C seems to be the optimal value for the commonly adopted evaporation temperatures (10°C-20°C). For a specific coating thickness, there is a critical switchover period where the best performance of dehumidification is obtained. Switchover period shorter than the critical time should be avoided, and elongating the switchover cycle helps balance the ability of the system to handle the sensible and latent heat loads at the same time. [ABSTRACT FROM AUTHOR]

In direct expansion ground coupled heat pumps (DX-GCHPs), the borehole heat exchanger (BHE) plays a major role, as it is the component where heat transfer between the ground coupled heat pump (GCHP) and the soil occurs. To address the current challenges of using CO2 as refrigerant in DX-GCHP systems, theoretical and experimental investigations were performed on a single U-tube vertical direct expansion BHE using CO2 as the refrigerant. First, an experimental setup of a transcritical CO2 DX-GCHP built at CanmetENERGY laboratories was used to monitor, collect, and analyze data for refrigerant flow, pressure drop, and ground temperature along the boreholes. The CO2 evaporation process in the BHE for different mass flow rates per borehole was then experimentally analyzed. Later, a previously developed theoretical model of the BHE was validated using a set of experiments collected with the current test apparatus. Finally, the model was used for the analysis of the effect of the following parameters on the performance of the BHE: (i) the refrigerant flow rate, (ii) the borehole wall temperature and average fluid temperature over the test period during the evaporation process, (iii) the pressure drop, (iv) the required power for CO2 circulation, and (v) the evaporating temperature change in the BHE. [ABSTRACT FROM AUTHOR]

For evaluating the heat transfer correlations from literature in designing helical coil evaporators for low-temperature organic Rankine cycles (ORC), an inverse evaporator design methodology is used. This is done by taking four already performed measurements with different working fluid mass flow rates (0.21–0.23 kg/s) and saturation pressures (1.93–3.05 MPa, reduced pressures of 0.51–0.82) at the helical coil evaporator (66 m long) in an already operational experimental solar/thermal ORC system as real-case references for the evaporator inverse design's boundary conditions. R-404A is considered as the working fluid. The heating water inlet temperature is changed between 353 K and 373 K. The total length is inversely calculated via 15 helical coil two-phase heat transfer correlations for each case. Their end designs are compared with the actual length. Results show that the correlations can be used interchangeably for designing helical coil evaporators for low-temperature ORCs since the heat transfer resistance is dominant on the shell-side. For evaluating the sensitivity of these results, a secondary analysis was made by means of changing the shell-side correlation's accuracy from its initial value of 10% to more accurate 7%. Predictions with several correlations design shorter heat exchangers at reduced pressures less than 0.7. [ABSTRACT FROM AUTHOR]

The present work deals with the development of thermodynamic model of low temperature basic Organic Rankine Cycle (ORC) system and a chevron plate heat exchanger evaporator sub-model using Engineering Equation Solver (EES). Work output is evaluated using the ORC thermodynamic model, while the evaporator sub-model calculates the total surface area of the heat exchanger. Using these mathematical models, the effect of evaporation pressure, expander inlet temperature and pinch point temperature difference (PPTD) on the network output and evaporator cost are studied. In addition to this, the effect of plate spacing and plate width of chevron plate heat exchanger on pressure drop and evaporator cost are analyzed in detail. Finally, thermodynamic and geometric optimization is carried out using genetic algorithm to identify the optimum parameters at which the network output is maximized and pressure drop in the evaporator is minimized. Sensitivity analysis showed that optimum evaporator pressure existed at which network output is maximum. Thermodynamic optimization showed that work output was maximum (5.03 kW) at evaporator pressure of 5.77 bar. No improvement in the work output was seen with increase in PPTD and expander inlet temperature. Increase in plate width and plate spacing led to increase in evaporator cost and decrease in pressure drop. [ABSTRACT FROM AUTHOR]

Highlights • A new Constructal law-based design is presented for cross-flow heat exchangers. • Compared to cross-flow design, the thermal performance is increased as much as 17%. • Compared to cross-flow design, the cost is decreased as much as 14%. • The method is computationally very efficient and can be used in preliminary design. Abstract With recent advancements in advanced manufacturing techniques, it is now possible to fabricate complex geometries that take advantage of well known principles of heat transfer. Therefore, unconventional configurations to enhance effectiveness beyond conventional designs can now be considered for practical application. Thermal performance and overall cost of a new design of heat exchangers in counter cross-flow configurations are studied using a simplified but accurate computational method. The new heat exchanger design was introduced and studied previously for a cross-flow configuration by Sabau et al. (2016, 2018). This new design concept uses multi-scale configurations with successive plenums for the working fluid. At the smallest scale the tubes are sized to be equal to the hydraulic entrance length of the inside fluid, in accord with constructal design. Results indicate that compared to the earlier cross-flow configuration, the counter cross-flow arrangement improves the thermal performance of the heat exchanger by as much as 17% and lowers the total cost by as much as 14%. [ABSTRACT FROM AUTHOR]

Graphical abstract This paper presents an experimental investigation of heat transfer performances including heat transfer coefficient along flowing direction, log-mean temperature difference and differential pressure of pure R123 and four volume concentrations 20 nm Al 2 O 3 -R123 nanorefrigerants with various volume concentrations 0.03%, 0.13%, 0.18%, 0.23% flowing inside a double-tube counter flow heat exchanger of an organic Rankine cycle system under the conditions of various heat source temperatures and flow rates. A comparison has been made between pure R123 and four kinds of nanorefrigerants to find out the influences of different nanoparticles volume concentrations on local heat transfer coefficient and variation tendency (straight lines) along flowing direction in evaporator. By using polynomial fitting where polynomial order is 3, the polynomial fitting curves (dash lines) and the optimum of volume concentration has been acquired, which indicates the loading carrying capacity in each operation condition. Highlights • Effect of Al 2 O 3 nanoparticles on R123 heat transfer performances. • Variation tendency along flowing direction in evaporator of ORC. • Polynomial fitting curve. • Comparison of differential pressure and log-mean temperature difference of R123 and nanorefrigerants. Abstract This paper presents an experimental investigation of variation tendency of heat transfer coefficient, log-mean temperature difference and differential pressure of pure R123 and 20 nm Al 2 O 3 -R123 nanorefrigerants with four various volume concentrations, 0.03%, 0.13%, 0.18%, 0.23% flowing inside the evaporator of organic Rankine cycle system under the conditions of various heat source temperatures and flow rates. Heat source temperatures are in the range of 50–90 °C at an interval of 10 °C, and heat source flow rates are 0.7 m3/h, 1.3 m3/h and 1.8 m3/h. Results show an increment of heat transfer coefficient along flowing direction for both pure R123 and four Al 2 O 3 -R123 nanorefrigerants with the increment of heat source temperature and flow rate, and that of four nanorefrigerants are higher than that of pure R123. There is no optimum value of heat transfer coefficient when operation condition is changed for the loading carrying capacity increasing with the increasing intensity of operation condition. Meanwhile, suspending nanoparticles and increasing heat source temperature can change the variation tendency of heat transfer coefficient along flowing direction except pure R123 and 0.18 vol% nanorefrigerant. In addition, 0.13 vol% as a whole is the optimum volume concentration for both log-mean temperature difference and differential pressure. [ABSTRACT FROM AUTHOR]

Highlights • A numerically efficient but accurate model for frosting evaporators is developed. • The enthalpy-based reformulation and linearization method are employed • Simulation results on flat plate and finned-tube heat exchangers are provided. • It is 8 to 20 times faster than reference models but withcomparable accuracy. Abstract Growth of a frost layer on an evaporator surface due to low evaporator temperature as well as moisture contained in surrounding air deteriorates performance of a refrigeration system significantly and requires significant energy for defrost. Many studies have been performed to model the heat and mass transfer phenomena in an attempt to have insight and accurate prediction. However, many models form nonlinear algebraic differential equations which require iterative numerical solvers. Computationally efficient but accurate models are needed in order to evaluate overall system performance. The objective of this paper is to introduce a modeling approach to overcome the problem. A solution strategy based on an enthalpy-based reformulation and linearization method will be presented. Comparisons of the proposed and detailed model results for both flat plate and finned tube heat exchangers are provided. The proposed modeling approach is around 10 times faster than reference models while maintaining comparable accuracy. [ABSTRACT FROM AUTHOR]

Highlights • A method for the determination of defrosting start time is proposed. • Frost volume and blocking ratio are calculated from experimental data. • Frost volume calculation method is verified by image processing of the evaporator. • This method accurately determines the timing of deterioration of the system. Abstract Naturally, frost forms at fin-tube evaporator surface when refrigerant below 0°C flows inside the heat exchanger. Frost tends to block the evaporator, hinders efficient heat exchange and degrades the performance of the system. The determination of the start time for defrosting is one of the essential issues that most of air heat pumps are confronted with because it is directly connected to energy consumption. Many types of sensors can be installed for frost detection but they are usually not cost-effective or related logic is not so physically presented. In this study, a novel frost volume calculation method is suggested in order to determine the start time for defrosting in an air-to-air heat pump system. The detailed explanation of the method is presented and verified with experimental data. With this method, the start timing of defrosting can be appropriately judged. At the time of defrosting, the heating capacity decreased from 16.2 to 39.3% and the COP reduced from 12.3 to 26.4% compared with maximum values. The blocking ratio from image processing at the time was between 0.83 and 0.97. [ABSTRACT FROM AUTHOR]

The performance and thermo-sustainability indicators (TSI) of non-hybrid organic Rankine cycles (ORCs) are considered at varying evaporator pressure (EVP) and heat source temperature (HST). The ORCs structures include the ORC-basic, ORC-internal heat exchanger (ORC-IHE), ORC-turbine bleeding and ORC-turbine bleeding/regeneration. The TSI considered exergy efficiency, waste exergy ratio (WER), exergetic sustainability index (ESI) and environmental effect factor (EEF). The calculated overall exergy efficiency varies from 35.35 to 47.23 for evaporator pressure between 2 and 3 MPa and 49.02 to 52.71 for HST between 500 and 650 K. Additionally, WER, ESI and EEF were found to vary from 0.253 to 0.278, 1.15 to 1.87 and 0.547 to 0.82, respectively while ESI decreases for all variants of HST. The results indicate high ESI and low WER in ORC-IHE and the ORC-turbine bleeding /regeneration for all variations in EVP and HST. [ABSTRACT FROM AUTHOR]

Thermal distillation systems are widely used in the evaporation concentration industry, and the steam is the heating medium of these systems. However, due to the low density of the steam, the volume of equipment and piping is large. Especially in the mechanical vapor recompression (MVR) systems, high impeller exit line speed often occurs in steam compressors, which will cause compressor blades failure easily. In order to solve this problem, an experimental system called a vapor compression heat pump combined with double‐stage forced‐circulation evaporators (VCHP‐FCE) was developed. The system incorporates the refrigerant, R‐22, which offers an energy recycling benefit. R22 is used as the heat resource and has a higher energy‐carrying ability per unit volume than steam. The advantages relative to traditional thermal distillation system include a smaller device size, compact structure and no need for steam resource. Experimental facilities were established to analyze the thermal parameters and performance of the proposed system. The preheating temperature, which was correlated with the refrigerant's evaporation temperature, was the decisive factor affecting the system. A positive correlation was identified between the system's water production rate and the preheating temperature. Based on the first and second laws of thermodynamics, we analyzed the energy balance and the exergy loss of the system. The proportion of energy loss in each unit and the complete system exergy efficiency as a function of the environmental temperature were determined, which together indicated directions for system improvement. A vapor compression heat pump combined with double‐stage forced‐circulation evaporators (VCHP‐FCE), was developed. The system incorporates the most commonly used refrigerant, R‐22, which offers an energy recycling benefit. This refrigerant is used as the heat resource and has a higher energy‐carrying ability per unit volume than steam. [ABSTRACT FROM AUTHOR]

• Heat exchangers in which phase changes occur require simulation models that realistically capture the resulting temperature profiles and heat-transfer rates. • Two industrially important process examples are studied: a steam generator heat exchanger and the two heat exchangers in a compression refrigeration system. • The reality of the process is compared to the approximate simulation implementation. • Effective control structures are developed and tested. The heat exchanger is probably second only to the pipe as the most widely used piece of process equipment. A typical chemical plant has hundreds of heat exchangers that transfer heat from a hot source into a cold sink. The analysis of heat transfer is straight-forward when both the hot and the cold streams do not undergo any change in phase during the heating and cooling. However, the analysis gets more complex when there is vaporization or condensation of either of the streams. Now latent heats are involved and temperatures do not change continuously with axial location. Vaporizers, reboilers, evaporators and condensers are common examples of heat transfer with phase changes. The purpose of this paper is to demonstrate how realistic simulation models can be developed that capture the steadystate and dynamic heat-transfer differential temperature driving forces, vapor-liquid fractions and heat-transfer rates for these important and widely applied exchangers. [ABSTRACT FROM AUTHOR]

Understanding the bubble regimes is a fundamental step toward conducting heat transfer enhancement. The non-invasive measurement of mixing inside a direct-contact heat transfer process, using a direct video imaging technology, provides powerful opportunities for characterising the visual observations of the phenomena and quantifying the process complexities previously. Experimental bubble shape feature parameters were obtained by means of the photographic recording technique for a direct-contact evaporator. Four design factors with three levels respectively were analysed for the mixing system that involves the exchange of heat between two immiscible fluids (continuous and dispersed phases). Using the Ripley’s K function, new results are presented for two-phase flow mixing which can distinguish differences in the mixing behavior of dispersed phase. In all cases considered, quantitative comparisons of the evolution curves representing different experimental conditions were conducted with reported experimental data. Following the local mixing curve, the current results can also be processed to provide the mixing time found to be in good agreement with available data. The relationship between shape feature parameters of bubbles and volumetric heat transfer coefficient was found to be highly independent on experimental design parameters. [ABSTRACT FROM AUTHOR]

This paper presents a derivation of design guidelines for plate heat exchangers used for evaporation of zeotropic mixtures in heat pumps. A mapping of combined heat exchanger and cycle calculations for different combinations of geometrical parameters and working fluids allowed estimating the trade-off between heat transfer area and pressure drops on the thermodynamic and economic performance indicators of the cycle. Compressor running costs constituted the largest cost share, and increased due to a steep decrease of the heat pump coefficient of performance at high refrigerant pressure drops. It was found that the pressure drop limit leading to infeasible designs was dependent on the working fluid, thereby making it impossible to define a guideline based on maximum allowable pressure drops. It was found that economically feasible designs could be obtained by correlating the vapour Reynolds number and the Bond number at the evaporator inlet as Re V − 0.42 Bd 0.26 ≈ 0.040 . The use of the proposed guideline was illustrated for the mixture Propane/Iso-Pentane (0.5/0.5), leading to evaporator designs with net present values deviating maximum −4.4% from the best value found in the mapping. The presented methodology can be applied in different scenarios to develop similar guidelines, thereby decreasing the cost of combined cycle and component optimizations. [ABSTRACT FROM AUTHOR]

Falling film, shell-tube type evaporators are commonly used heat exchangers for the production of fruit juice concentrate. The main problem in the design of the exchanger is a reliable estimation of wall heat transfer coefficients for all effects in real operating conditions. Most literature sources for the overall heat transfer coefficients are based on laboratory measurements, where the tubes are usually short, no fouling exists and the flow rate is carefully adjusted. This paper shows the heat transfer estimated in real industrial operating conditions, compared to literature sources. Paper is based on the author's own experience in designing and launching several evaporators for juice concentrate production into operation. As a summary, the design heat transfer coefficients are provided with relation to sugar content in juice concentrate. [ABSTRACT FROM AUTHOR]

This study is developed to evaluate the possibility of R1234yf to be a drop-in replacement for a pre-designed system with R134a, including the characterization of control system in a vapor compression system. The control algorithms are required for regulating the refrigerant flow rate into the evaporator and their control parameters depend intrinsically on refrigerant fluid. This paper presents a comparison of the effects of R134a and R1234yf refrigerants on the transient response of an evaporator operating with water as secondary fluid (counter-flow). The dynamic heat exchanger distributed model combines principles of thermodynamics, heat and mass transfer with empirical correlations, which are proposed in the literature, in order to compute the void fraction, pressure drop and heat transfer in two-phase flow. Experimental tests are carried out on a fixed refrigeration system for cooling capacity less than 3 kW, with constant condensation conditions, variable evaporating temperature and compressor speed to validate the model. The results demonstrated that a good agreement between simulation results and experimental data was achieved. The evaporation temperature data were within ± 1   °C error band. Furthermore, comparative simulations between the refrigerants R-134a and R-1234yf exhibited a similar dynamic behavior during a step at the inlet mass flow rate, and the gain and time constant for superheat control were not considerably modified. Therefore, the refrigerant changing does not demand many changes in the expansion device control strategy. [ABSTRACT FROM AUTHOR]

Heat exchangers are common components of many industrial, residential, and automotive systems. In automotive, heat exchangers are employed in engine cooling and air conditioning to transfer the undesired heat from the engine or the passenger compartment as a condenser, evaporator, inter-cooler, heater core, radiator, and oil cooler. Since, the thermal performance of heat exchangers influences the underhood automotive thermal management system, it is essential to characterize their dynamic response in general and specifically, when a sudden change to their operating parameters occur. Experimental measurements provide the scope for in depth understanding of the dynamic behavior of heat exchangers with respect to various parameters. This study investigates a liquid to gas cross flow meso heat exchanger subjected to transient conditions. Step variations in the hot fluid inlet temperature in the absence of mass flow rate disturbances was considered for 5 different levels ranging from 1.5 to 3.0, while other operating parameters are kept constant. The effect of step change on the thermal performance of a heat exchanger is demonstrated by both fluids outlet temperatures, heat transfer rate, effectiveness, and heat balance error. Results obtained show a faster response of the cold fluid outlet temperature compared to the hot fluid, however, an adverse effect is noticed for the heat transfer rate. Higher step changes in temperature lead to higher heat transfer rate, as expected; though, the hot side exhibited more heat transfer and higher effectiveness than the cold side. The hot fluid heat transfer rate and effectiveness display a non-linear increase reaching a peak, after which, it drops down at a slower rate compared to the cold fluid steady increase. A significant effect on normalized exit temperatures, heat transfer, and effectiveness at low step changes while it diminishes at higher step changes. The key element of this work is the use of meso heat exchanger to examine the transient conditions effect on its performance. This work covers a wide range of temperature step changes and aims to enrich the limited experimental database of dynamic response of heat exchangers. The outcome of this work is of interest to designers for thermal testing, characterization, and performance enhancement of compact heat exchangers. [ABSTRACT FROM AUTHOR]

Performance of a frost-free refrigerator may be deteriorated by frost accumulation on its evaporator surface. The objective of this study is to investigate the feasibility of using an enthalpy exchanger (EE) to reduce the frost accumulation on evaporator surface in frost-free refrigerator. The influence of EE on air circulation and the change process of air humidity is analyzed, and it is indicated that the application of EE is capable to decrease the amount of water vapor entering the evaporator and consequently reduce the frost accumulation on evaporator surface. The design process of EE in frost-free refrigerator is illustrated, and an optimal type of EE is presented. An experiment on a frost-free refrigerator shows that the application of EE decreases the frost accumulation by 18.8%. In addition, effects of EE on other main performances of frost-free refrigerator besides the frost accumulation are evaluated. [ABSTRACT FROM AUTHOR]