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This work investigated comprehensively the impacts of non-ideal optical factors, including incident angle, sunshape and optical errors on the performance of the parabolic trough collector (PTC). Each optical factor was defined based on their geometrical principles. It was revealed that the heat flux distribution distorted by optical factors was the main cause of changing performance of the PTC. The temperature distribution was completely dependent on the heat flux distribution. The incident angle caused cosine loss and end loss, which respectively reduced the effective incident solar radiation and produced a near-zero heat flux section at one end of the absorber. Based on the effective incident solar radiation, the collector efficiency was reduced by 41.11% with the incident angle increasing from 0° to 60°. Larger circumsolar ratios produced more uniform circumferential temperature distribution, while reduced greatly the collector efficiency. The specularity error and tracking error affected slightly the receiver's safety, while the slope error reduced obviously the threat to the receiver. When specularity error was small enough (

A model based on ASNYS CFX was established to explore the pressure drop and heat transfer characteristics of condensation flow with hydrocarbon mixtures in a spiral pipe under static and heaving conditions, whose deviations are within ±15% compared to experimental ones. It considered the effects of motion parameters, operating parameters and structural parameters on frictional pressure drop and heat transfer coefficient for three hydrocarbon mixtures. The ranges of mass flux, vapor quality, heat flux, saturation pressure, hydraulic diameter, curvature diameter and inclination angle were 200–800 kg/(m2·s), 0.3–0.9, 10–20 kW/m2, 2–4 MPa, 6–14 mm, 1.6–2.4 m and 6–14°, respectively. The results demonstrate that methane/ethane mixture has the best heat transfer and the smallest pressure drop among three mixtures. At static condition, as the vapor quality and mass flux increase and the saturation pressure and hydraulic diameter decrease, there are evident increases in heat transfer and pressure drop, whereas the impacts of curvature diameter, inclination angle and heat flux on them are insignificant. Meanwhile, the heaving motion affects pressure drop and heat transfer and makes it show better overall heat transfer performances, where the comprehensive heat transfer factors (CHFs) of 83.33% data points are more than 1. Further, the heaving effects increase with the increasing heaving amplitude and the decreasing heaving cycle.

Enhancing the heat transfer in the absorber tube improves effectively both the thermal efficiency and the structural safety of the parabolic trough receiver (PTR). This study proposed a novel unilateral spiral ribbed PTR (USR-PTR) for enhancing the heat transfer inside the absorber. The flow and heat transfer performance of the proposed USR-PTR was studied numerically using CFD tool coupled with Monte Carlo Ray Tracing (MCRT). A detailed comparison between the USR-PTR and the conventional straight and smooth PTR (CSS-PTR) was conducted in terms of both thermal and hydraulic performance, revealing that fluid disturbance, rotational flow and local longitudinal vortexes induced by the discontinuous spiral ribs are the three main causes of heat transfer enhancement in the USR-PTR. In most cases of the discussed flow rates, the overall performance of the USR-PTR is better than that of the CSS-PTR. The circumferential temperature difference of the USR-PTR can be reduced by up to 25% compared to the CSS-PTR. The structural parameters of the spiral rib influence greatly the performance of the USR-PTR. It is revealed that the improvement of the overall performance of the USR-PTR by changing individually the rib structural parameters, including pitch interval, rib height, corner radius and spiral angle, can be up to 12.5%, 9.8%, 10.8% and 30.1% respectively.

In recent decades, multi-split air source heat pump (M-ASHP) unit has been widely used for space heating. Similar to the split-ASHP unit, frost would accumulate on its outdoor coil surface under frosting condition, which significantly deteriorates its heating performance. In the previous study, a novel reverse-cycle defrosting method based on phase change energy storage was developed and investigated, and the study demonstrated that the novel defrosting method could significantly improve defrosting performance. To provide guidelines for designing the M-ASHP units with phase change energy storage, it is necessary to investigate the characteristics of heating and energy storage for M-ASHP units, which is presented in this paper. Experimental results showed that there was no frost on outdoor coil when only the indoor unit with heating capacity of 7.1 kW was turned on under frosting condition. In addition, the energy storage period increased as the number of operating indoor unit increased, namely the load rate. Furthermore, the optimal starting time of the energy storage operation was around 10 min after the start-up of M-ASHP unit. Even though, compared with the conventional heating mode with three turned-on indoor units, the energy storage mode shortened the heating period by 8 min and decreased the average heating capacity and COP by 4.4% and 1.9%, respectively. The above negative impacts of energy storage operation on the heating performance of the M-ASHP unit could be neglected by considering its positive impacts on defrosting performance, for a periodic heating-defrosting condition.

There were very limited modeling studies are around a phase change material based double-spiral coiled heat exchanger, due to the difficulties of model development and the lack of mature simulation tools. When a phase change material based double-spiral coiled heat exchanger applied in an air source heat pump for defrosting, its model development becomes more complicated. To solve this fundamental problem, a mathematical model for such heat exchanger was developed, as well as the numerical solution method proposed. To improve the convergence and the speed of numerical simulation of the phase change material’s heat transfer process, both of special treatments in space and time were conducted. According to the circular shape boundary, an implicit solution method was used to reduce the number of grids so that computation became convenient. Heat storage and release processes of this model were finally experimentally validated in a specially built heat pump system during frosting and defrosting, respectively. The deviation conditions are demonstrated acceptable with the mean average temperature differences at the small range of −0.71–1.64 °C. Potential uses and limitations of the modeling work are also further discussed. Contribution of this work are meaningful for the optimization of heat pump and any other systems, where applied a phase change material based heat exchanger.

In order to explore condensation flow and heat transfer characteristics in the tube side of spiral wound heat exchange (SWHE) used in floating liquefied natural gas (FLNG) field, a model was established to simulate methane condensation upward flow in a spiral pipe at different sloshing conditions, such as heave, roll and pitch conditions. The simulated results coincide with experiment ones, and the deviations are within ±15%. The effects of various parameters on frictional pressure drop and heat transfer coefficient were discussed in pipes with curvature ratio of 0.003–0.007. The ranges of Reynolds numbers in vapor and liquid flows are respectively 154,944–697,251 and 18,190–127,331. It was found that at static conditions, both frictional pressure drop and heat transfer coefficient increase with the increase of mass flux and the decrease of saturation pressure and hydraulic diameter; with the increase of vapor quality, the heat transfer coefficient first increases and then decreases whereas the frictional pressure drop continuously increases; the effects of curvature diameter are slight. Meanwhile, in most cases, the sloshing motions can cause a remarkable heat transfer enhancement as well as a small decrease of frictional pressure drop. Further, the effects of heaving motion on flow and heat transfer are more obvious than the others, with the average drag reduction and heat transfer augmentation of 1.51% and 15.70%, respectively. These results are helpful for the understanding condensation characteristics in the spiral pipe at sloshing conditions, and for the design of SWHE used in FLNG field.

With the increase in data traffic, high energy consumption of cooling systems in data centers are rising continually and rapidly. Thus planning energy is more and more important to minimize resources consuming. The equipment in data room can produce vast amount of heat which must be removed, and waste heat recovery is an effective means of saving energy. In this paper, a simulation of the annual dynamic air conditioning load of buildings was conducted through Designer's Simulation Toolkit (DeST) to get the cooling and heat load index, and a data center in Harbin was taken as a case study to evaluate the energy-saving effect. The results indicated that the annual cumulative cooling load was far greater than the annual cumulative heat load, so it has great potential for heat recovery. Then a system that made use of waste heat from data rooms to serve subsidiary buildings was proposed. It could fully satisfy the heat demand in data centers when equipment in data rooms all run. Meanwhile, the heat recovery system has a better economic viability when compare with the air source heat pump system. Therefore, using the heat recovery system can improve the energy efficiency and realize the energy saving.

In this paper, the condensation heat transfer of hydrocarbon mixtures refrigerant (methane/propane) flowing in a helical tube with 10 mm hydraulic diameter was investigated by experiments and numerical simulations. The experiments were conducted at mass flux of 200–400 kg/(m2·s), saturation pressure of 0–4Mpa and heat flux of 4.8–5 kW/m2. Also, the effects of mass flux, vapor quality, and saturation pressure on heat transfer coefficient were studied. It was observed that the heat transfer coefficients increased with the increase of mass flux and vapor quality, while the heat transfer coefficients generally decreased as the saturation pressure increased. Meanwhile, annular flow(half-annular flow, annular flow) and Non-annular flow patterns (slug flow, stratified flow, wavy flow, transition flow) were observed during the condensation of methane/propane mixtures refrigerant. And a new flow pattern transition line for hydrocarbon refrigerants condensation process was proposed. The condensation heat transfer coefficient data were compared with several existing correlations, and a modified heat transfer correlation for different flow patterns was proposed, which predicted the results well with a mean absolute error of 10.03%.

A numerical model of condensation heat transfer and frictional pressure drop in a circular tube was developed and validated. In order to facilitate the model validation, the physical model was built according to the experiment condition in the existing literature. In the numerical models proposed in this work, the following models were involved: volume of fluid (VOF) multiphase model, Lee’s phase change model and Reynolds Stress turbulence model. The importance of boundary-layer mesh size was analyzed, and the effects of entrainment, turbulent Prandtl number and gas-liquid interface turbulence damping on the simulation results of heat transfer coefficient (HTC) and frictional pressure drop (FPD) were evaluated and modified. The numerical model was validated with Neeraas’s experimental data and Steiner's correlation results at various mass fluxes, vapor qualities, pressures and fluids. The simulation results of void fraction showed good agreement with Steiner's correlation; the HTC and FPD showed good agreement with Neeraas’s experimental data, which indicated the model developed in this work has good universality and accuracy.

Helical tube has been widely used in liquefied natural gas (LNG) liquefying applications. However, studies on the heat transfer performance of hydrocarbon mixture refrigerant in a helical tube have rarely been conducted. In this paper, condensation heat transfer characteristics of hydrocarbon mixture refrigerant in a helical tube were investigated numerically. The tests were conducted at saturation pressure of 2–4 MPa, mass flux of 100–400 kg/(m2·s) and vapor quality 0–1. The numerical model was well verified by experimental data. And the effects of different structural parameters (tube diameter, helix angle and curvature diameter) and operating parameters (mass flux, heat flux and saturation pressure) on the heat transfer of hydrocarbons refrigeration were considered. The results showed that the tube diameter and mass flux had significant effect on condensation heat transfer coefficient. Meanwhile, stratified flow, half-annular flow and annular flow were observed during the condensation of methane/propane mixtures refrigerant by simulations, and a new flow pattern transition line for hydrocarbon refrigerants condensation process was proposed. Finally, the simulation results were compared with several well-known condensation heat transfer correlations. The research results will provide some constructive instructions for more effective design of LNG heat exchanger.

Fouling deposit is a common issue on the heat transfer surface caused by the impurity of working water. Many researchers conducted experimental test to study the relationship between the fouling thermal resistance and operating parameters, such as water quality, tube geometry, and liquid velocity, targeting at developing the accurate correlation of fouling thermal resistance on heat transfer tubes. The accurate test of fouling thermal resistance is critical for investigators. In fouling test, with the fouling deposit on the internal surface, both the liquid (water) velocity through the tube and the heat flux of the test tube deviated automatically. Although testers usually tried to adjust the water velocity and heat flux back to the original point, it is hard to be realized, thus the water velocity and heat flux deviated somehow inevitably. In fact, the variations of water velocity and heat flux would cause the change of overall thermal resistance of test tubes, which should be separated from the change caused by fouling deposit. This process could be named as “decoupling”. This paper analyzed the effect of deviations of water velocity and heat flux on the test results of fouling resistance quantitatively based on experimental test, and a decoupling method and formulas were developed. One set of accelerated fouling test was conducted and result shows the fouling resistance with decoupling and non-decoupling had a maximum difference of 0.000002124 m2 K/W for tube 1, and 0.000002363 m2 K/W for Tube 2, 0.000001316 m2 K/W for tube 3.

Coil-wound heat exchangers (CWHEs) have been widely used in natural gas liquefaction plants nowadays. But few studies on the condensation heat transfer and frictional pressure drop of mixed hydrocarbon refrigerants inside the tubes of CWHEs have been conducted under practical conditions. In this study, numerical simulations were carried out to investigate the condensation flow and heat transfer characteristics of mixed hydrocarbon refrigerants inside the tubes of liquefied natural gas (LNG) CWHEs under practical operating conditions. A combinational numerical method based on computational fluid dynamics (CFD) simulation and modified Silver method was proposed to predict the condensation heat transfer coefficients of mixtures. The numerical results showed good agreement with experimental data and calculation results by the empirical correlation in the literature. The relative errors of void fraction, heat transfer coefficient and frictional pressure drop were predicted within ±5%, ±15% and ±15% respectively. The simulation results indicated that the heat transfer coefficients of mixed refrigerants varied slightly with the increase of vapor quality, but significantly with the increase of mass flux. And the frictional pressure drop increased with the increase of both vapor quality and mass flux. This study is instructive to the design and optimization of CWHEs used for natural gas liquefaction process.

In this paper, the condensation heat transfer and pressure drop characteristics for hydrocarbon mixture upward flow in smooth and enhanced spiral pipes were numerically investigated. The numerical model was established and verified by experimental results in literatures. It discussed the influence of geometrical parameters on condensation heat transfer and pressure drop in three kinds of enhanced spiral pipes, which contained square corrugated pipe, sinusoidal corrugated pipe and spiral grooved pipe. The results indicate that in enhanced spiral pipes, both frictional pressure drop and heat transfer coefficient increase with the rise in corrugation (groove) height and the decrease of corrugation (groove) pitch. Meanwhile, compared to the smooth pipe, the augmentation on heat transfer for square corrugated, sinusoidal corrugated and spiral grooved pipes are 0.934–2.052, 1.103–2.216 and 1.206–1.804 times, respectively, while the increase of frictional pressure drop are 1.805–10.930, 1.272–7.176 and 0.851–3.587 times, respectively. Besides, the comprehensive heat transfer enhancement factor (CHF) was introduced to evaluate the overall heat transfer performance of enhanced pipes. It is found that as the corrugation (groove) height increases, the CHF increases in square and sinusoidal corrugated pipes but decreases in spiral grooved pipe; at the meantime, with the increase of corrugation (groove) pitch, the CHF first decreases and then increases in square and sinusoidal corrugated pipes while first increases and then decreases in spiral grooved pipe. The average CHFs for all square corrugated pipes, sinusoidal corrugated pipes and spiral grooved pipes are 0.872, 1.083 and 1.275, respectively. Moreover, the spiral grooved pipe with the relative groove height and pitch of 0.03535 and 5.0 shows the best overall heat transfer performance among others, whose average CHF can reach to 1.413. These results provide some instructions for the application of enhanced spiral tube in the design of spiral wound heat exchange (SWHE).

A model was established to explore the shell-side heat transfer characteristic of saturated boiling in spiral wound heat exchanger (SWHE), which was used for liquefied natural gas (LNG). The saturated boiling mechanism was investigated by ANSYS CFX based on the model which was validated by the experimental data, and the correlations for shell-side heat transfer coefficient of SWHE were improved, where mass flux G, heat flux q and vapor quality x were 40–100 kg / ( m 2 · s ) , 2,000–8,000 W / m 2 and 0.3–0.9, respectively. The results show that the deviations between simulation results and experimental data are within ± 13 % . According to the simulation results, the main boiling mechanism for shell-side saturated boiling heat transfer is forced convective, and the contribution of nucleate boiling contribution is slight (less than 0.77 % ). At the same time, the improved correlations are in good accuracy, the deviations between correlation calculation results and simulation results are within ± 25 % .

Spiral wound heat exchanges (SWHE) has been the most widely used in large-scale liquid natural gas (LNG) plants. However, few studies have been focused on the condensation heat transfer and pressure drop for hydrocarbon mixture refrigerant in SWHE tube side. In this paper, the condensation heat transfer and pressure drop characteristics for ethane/propane mixture upward flow in a spiral pipe were numerically investigated. The numerical model was established and verified based on the existing experimental results and new flow pattern observation experiments. It discussed the variation trends of void fraction, frictional pressure drop, heat transfer coefficient and heat and mass transfer resistance with various parameters, such as, vapor quality, mass flux, heat flux, saturation pressure and inclination angle. Comparing with the existing correlations, the results showed that Steiner’s correlation, modified Fuchs’s correlation, Boyko’s correlation could better predict the void fraction, frictional pressure drop, film heat transfer coefficient with mean absolute deviation of 6.08%, 10.67% and 13.06%, respectively. Meanwhile, modified Silver approach was used to modify the mixed effects of ethane/propane mixture on heat transfer. The study provides some constructive instructions to understand the condensation of zeotropic mixtures in the spiral pipe, and is helpful in designing more effective SWHE.

In order to explore the shell-side flow and heat transfer characteristics for spiral wound heat exchanger (SWHE) in floating liquefied natural gas (FLNG), a model was established to simulate the flow and heat transfer of shell-side superheated vapor flow under rolling working conditions. The effects of rolling parameters and working parameters were investigated. By comparing, the deviations between simulation results and experimental data were generally within ± 10 % . Under static working conditions, both frictional pressure drop gradient and heat transfer coefficient increased with the increasing mass flux, which were consistent with the predictions of calculation correlations. After that, an improved heat transfer correlation was proposed with an error of ± 5 % . Meanwhile, rolling motions can affect the shell-side flow and heat transfer characteristics of superheated vapor flow and lead them to show obvious periodicity. The influence of rolling period was smaller than that of rolling amplitude, and 90 % of the working conditions in FLNG SWHE can lead to the enhancement of heat transfer from −0.5 % to 2.6 % . These results can provide some instructions in the design and efficient running for FLNG SWHE.

A new algorithm for obtaining the critical tube diameter and intercept factor of parabolic trough solar collectors (PTCs) under the condition of tube alignment error has been developed theoretically, which, compared with the Monte Carlo Ray Tracing (MCRT) method, reduces the computing time remarkably. The results produced by the proposed method comply very well with the results obtained by MCRT. The critical tube diameters for different alignment errors can be precisely calculated using the algorithm, which can also be used to explain well the variation of optical efficiency. Effects of structural parameters on the optical performance are also discussed comprehensively. It is revealed that the offset direction that is perpendicular to the focus-edge connection line is most likely to cause rays-escaping. There exists an aperture width range (and also focal length range) in which the optical efficiency decreases with increasing offset angle, which is contrary to the conclusion presented in previous literature that the effects of X-direction offset (a = 0°) is greater than that in Y-direction (a = 90°). The proposed algorithm establishes the foundation for further geometric study on the coupling effects of multi-errors on PTCs’ performance, and can also be used for quick calculation and analysis in practice.

The maldistribuiton of gas-liquid mixtures has serious influences on the heat-transfer characteristics of plate-fin heat exchangers (PFHEs) and restricts the development of PFHEs in the floating liquefied natural gas (FLNG) field. To efficiently use PFHEs for FLNG, it is necessary to investigate the gas-liquid distribution at the entrance of PFHEs under sloshing conditions. In this study, an experimental gas-liquid distribution system was built, and the distribution characteristics of a PFHE distributor were investigated under sloshing conditions using air and water as mediums. The influence of sloshing form, sloshing amplitude, and sloshing period on the gas-liquid distribution is discussed with flow ratio m∗k and standard deviation STDk as the distribution index. The results showed that in the experimental condition the non-uniformity of two-phase mixture under sloshing conditions increase by 1.25–18.03% on the basis of steady condition with the increase of sloshing amplitudes and decrease with the increasing sloshing periods. The variation range of two-phase uniformity under mixed sloshing conditions is 14.8–27.9% on the basis of steady condition. These results provide constructive instructions for the design and safe operation of PFHEs in the FLNG field.

Spiral-wound heat exchanger (SWHE) is under sloshing working conditions when it is used on floating liquefied natural gas (FLNG), which can remarkably influence the efficiency of liquefaction and the stability of operation. In this paper, a numerical method for realizing the sloshing of SWHE shell side with different degrees of freedom (DOFs) was successfully proposed based on ANSYS software. The sloshing simulations were carried out, and the effects of sloshing motions on the shell-side flow and heat transfer characteristics were analyzed. By comparing, the uncertainty of simulation result was within ±10%. As a result, the shell-side flow and heat transfer characteristics were obviously influenced by sloshing motions and showed periodicity. The effects of pitching and heaving were more significant than that of rolling, and the effect of sloshing period was smaller than that of sloshing amplitude. At the same time, the fluctuations’ regularity of two-phase flow and heat transfer characteristics decreased with the increasing vapor quality.

A numerical investigation was carried out to study the condensation flow and heat transfer coefficient of methane/propane and ethane/propane mixtures in a helically coiled tube. Two-phase heat transfer simulations had been performed at saturation pressures of 0.3–3.7 MPa with mass flux of 200, 300, 400 kg/(m2 s) and heat flux of 5 kW/m2. The effects of mass flux, saturation pressure, vapor quality and component ratios on the heat transfer coefficient were examined. In addition, annular/non-annular flow patterns were obtained by numerical simulations and experiments, and a new flow pattern transitional line for annular/non-annular flows was proposed. Meanwhile, the results were compared with the existing condensation heat transfer correlations, and the improved heat transfer correlation was proposed and well coincided with the data with a mean absolute relative deviation (MARD) of 9.2%.

In order to explore the shell-side heat transfer characteristic for spiral wound heat exchanger (SWHE) in floating liquefied natural gas (FLNG) field, a model was established to simulate the heat transfer of shell-side film flow in real working conditions. The influence of rolling parameters and working parameters on shell-side heat transfer characteristic was investigated. For ethane, the results showed that the numerical model satisfies the accuracy requirement. In general, under static working conditions, the heat transfer coefficient increases with the increase of mass flux and the decrease of working pressure, but heat flux has little influence on it. These simulation results were consistent with experimental data and calculation results by correlations. And the heat transfer coefficient shows obvious periodicity under rolling working conditions. The rolling movement can enhance the heat transfer of shell-side film flow, and the influence of different parameters on heat transfer is different. These results will provide some instructions in the design and safe operation for SWHE in FLNG.

When the outdoor air temperature is low and relative humidity is high, frost will occur and accumulate on the outdoor coil of an ASHP unit, which may decrease the heating performances of the ASHP unit to a large degree. In addition, the defrosting performances of the commonly used defrosting method (reverse cycle defrosting) are poor due to the closed indoor air fan and insufficient energy stored in a conventional indoor coil. On the other hand, the conventional indoor coil based on forced convective heat transfer may cause strong draught sensation and dry eye problem and make users feel less comfortable than radiant heating terminals. Therefore, to improve the system defrosting performances and combine the merits of the conventional indoor coil and those of radiant heating terminals, a novel radiant-convective heating terminal was developed and integrated with an ASHP unit. The frosting and defrosting performances of the novel system were experimentally investigated, and the results showed that the average values of radiant panel surface temperature, outlet air temperature for the novel heating terminal, and system COP were 36.6 °C, 29.4 °C, and 2.78, respectively, during a 66-min frosting period (air temperature is 2.0 °C and relative humidity 84.0%). In addition, the ratio of the radiant heating capacity to natural convective heating capacity to forced convective heating capacity remained at around 1.5:1.0:7.4 during the whole frosting period. Furthermore, the experimental results demonstrated that the novel heating terminal could provide sufficient energy for the defrosting, and some energy could be provided for space heating through radiant and natural convective heat transfer during defrosting. The defrosting and resuming heating periods were 105 and 65 s, respectively.

Many studies have been carried out on frost distribution and growth and their impacts on the performances of finned tube heat exchangers (FTHXs), the most commonly used structure for the outdoor coils of air source heat pump units. However, in almost all the studies, ethylene glycol water solution was used as refrigerant, which made the investigating of frost distribution and growth difficult due to the uneven temperature distribution along the tube length. Furthermore, quantitative experimental data for the frost distribution and growth on FTHXs have been insufficient. Therefore, in this paper, a quantitative experimental study using R410A as refrigerant on frost distribution and growth on two FTHXs with different fin pitches is reported. The study results demonstrated that the frost accumulated on an FTHX could be divided into two parts, one accumulated on the edge of windward fins and the other on the surfaces of fins and tubes. Furthermore, the ratios of the frost mass accumulated on the edge of windward fins to that on the entire FTHX surface were 13.7% (60 min frosting duration) and 12.5% (120 min frosting duration) for the two FTHXs, respectively. The frost thickness and average frost densities in these two parts were also evaluated and compared.

In order to investigate the condensation heat transfer and pressure drop characteristics in tube-side flow of spiral wound heat exchanger (SWHE), a computational model based on two-fluid multiphase model, k-e turbulence model and thermal phase change model was established considering the gravity and surface tension. Also, a calculation formula for mean interfacial length between vapor phase and liquid phase was deduced and the wall contact angle was discussed. Based on the above model, numerical simulations were carried out on the condensation flow and heat transfer for propane upward flow in a spiral pipe. Compared with experimental data, it shows that the errors for the simulation results based on the new model are within ±15% under different operating parameters. Meanwhile, the heat transfer coefficient and frictional pressure drop are mainly influenced by vapor quality, mass flux and saturation pressure while the heat flux almost does not affect them. Comparing with the existing correlations, the improved correlations based on Boyko’s correlation and Fuchs’s correlation have been developed to predict condensation heat transfer coefficients and frictional pressure drop. These results will provide some constructive instructions for the understanding of condensation heat transfer and pressure drop characteristics in the spiral pipe, as well as the design and prediction of condensation heat transfer and pressure drop performance in SWHE.

In this paper, the characteristics of two-phase heat transfer and frictional pressure drop for hydrocarbon refrigerant propane condensation in a helical tube were experimentally investigated. A new experimental facility was designed to directly measure the wall temperature along a helical tube with the hydraulic diameter of 10 mm. The experimental conditions were set as: mass flux from 200 to 400 kg/(m2·s), heat flux from 1.4 to 9.6 kW/m2 and saturated temperatures from −40 to 27 °C. Also, the effects of mass flux and vapor quality on heat transfer coefficient and frictional pressure drop were studied. Compared with the experimental data, it showed that Shah correlation could well predict heat transfer coefficients within a mean deviation of ±20%, and Friedel correlation and Muller-Steinhagen & Heck correlation both well predicted frictional pressure drop with a mean deviation less than ±20%. These results will provide important instructions for the tube-side design of spiral wound heat exchanger.

Vapor compression air source heat pump (VCASHP) system has been widely applied due to its simple structure and low initial cost. However, there are a series of problems when the system is operated for space or water heating in cold regions, and a number of research groups have done much work to improve VCASHP systems for solving these problems over the last two decades. This review presents an update on the recent advances in VCASHP systems applied in cold regions. In this paper, the advanced systems are classified into three major types: single-stage, dual-stage and multi-stage compression systems. In addition, the dual-stage compression system is divided according to the number of compressors and separate loops into quasi-two-stage, two-stage and cascade compression systems. The heating performances and characteristics of each type of compression systems with different working fluids and intermediate configurations have been analyzed and summarized. Meanwhile, several issues of these advanced systems that impede their widespread applications in cold regions are proposed. It is hoped that both the reviewed contents and proposed issues may be useful to the research fellows working in this field.

Recently, with the rapid development of LNG (liquefied natural gas) industry, the utilization of LNG cold energy is becoming increasingly important because of the huge amount of energy released in the regasification process. LNGCW (LNG cold warehouse) system, as a common by-product of LNG, is often used with the benefit of energy saving. De facto, few comprehensive and quantitative researches on the LNGCW system are seen in open literature, thus it is necessary to analyze and evaluate the thermal performance of LNGCW system in depth. In this paper, some important results of regasification under different pressures were revealed, including LNG cold energy released, LNG cold exergy released and UA value of LNGHX (LNG heat exchanger). Secondly, a three-stage parallel LNGCW system was proposed and R23 was selected as the intermediate working fluid for this system. When the regasification pressure is 7.0 MPa, thermodynamic analysis showed that the LNGCW system exergy efficiency was 64.75%, and the COP was 1.41. Moreover, both the COP and exergy efficiency of the LNGCW system rose as the LNG regasification pressure increased. Finally, a qualitative analysis showed that LNGCW system as the subsequent stages was more favorable.

The influences of sunshape and incident angle on the optical performance of the parabolic trough solar collector (PTC) are investigated comprehensively based on Monte Carlo Ray Tracing (MCRT) method and theoretical analysis. This study adopts the acceptance-rejection sampling method, instead of the direct sampling method, for random variable sampling to reduce computing complexity. Both the energy proportion bounded by any angle span in the optical cone and the end loss caused by any incident angle are derived theoretically. It is revealed that the sunshape has great effects on the optical performance of a PTC, which should be taken into consideration in practice. The geometrical configuration of the PTC, especially the absorber tube diameter, should be determined to meet the energy requirement based on local sunshape (circumsolar ratio) conditions. It is also found that the end loss, caused by the incident angle, weakens the optical efficiency. Larger aperture width and smaller absorber diameter will cause greater end loss for constant incident angle. When the absorber length is long enough, the effect of incident angle will be negligible. In a range of small focal lengths, the optical efficiency increases with the increase of focal length, and then decreases constantly with further increasing focal length. Findings in this paper can be used as reference or guidance for designing and optimizing PTC’s structure in practice.

The recovery of condenser heat is concerned one of the most effective methods to curb energy consumption in residential dwellings. Aiming at recovering the condenser heat of domestic air conditioner, this paper experimentally studied a multi-functional domestic air conditioner with integrated water heater (MDACWH) which can effectively provide space – cooling and domestic hot water simultaneously. The dynamic operation characteristics, such as hot water supply and energy efficiency were tested to verify the availability of the MDACWH. The results showed that the MDACWH can effectively heat the domestic hot water without losing its cooling capacity. It was also found that with the use of MDACWH, the coefficient of comprehensive energy performance of the MDACWH was about 1.58 times higher than that of the unmodified experimental unit. Furthermore, the water-heating time was shorten remarkably from 128.5 min to 22.0 min. The novel domestic air conditioner, compared with the unmodified initial prototype, can be more practical and provide significant energy savings in space-cooling and hot water supply.

The optical performance of a parabolic trough solar collector (PTC) is studied comprehensively based on Monte Carlo Ray Tracing method (MCRT) and theoretical analysis. The MCRT models are established, and the theoretical equations of several critical parameters are derived firstly. And then the effects of different geometrical parameters on the optical performance are discussed in detail. It is revealed that the distribution of local concentration ratio around the absorber tube changes greatly, and cannot be divided into four parts as previous studies showed for some special parameter conditions. The theoretically derived parameters explain very well the different properties for the critical conditions. The variations of those parameters with different geometrical configurations are further displayed. Accordingly, the optical properties for different critical parameters are discussed. The size relationship between the reflected light cone and the absorber diameter affects the optical efficiency significantly because of the rays escaping effect. Practically, the absorber diameter should be larger than the size of the reflected light cone to avoid great optical loss caused by rays escaping. All the findings in this paper establish the foundation for further research on the optical-to-thermal energy conversion in the PTC system, and provide a reference for designing and optimizing PTC’s structure.

When air source heat pump (ASHP) units operate in heating mode at low temperatures in high humidity environments, frost forms and accumulates on the surface of its outdoor coils. This frost layer adversely degrades the operating efficiency of ASHP units rapidly, and can even result in sudden shutdown. Due to this operational problem, many researchers have conducted studies on frost-retardation measures. Since the frost that is present on the ASHP unit after shutdown has to be removed, defrosting becomes necessary. Among several reported defrosting methods, such as compressor shutdown defrosting, electric heating defrosting, hot water spray defrosting and hot gas bypass defrosting, the most popular method is reverse cycle defrosting. In addition, defrosting initiation and termination control play an important role in the frosting and defrosting cycle. Due to a scarcity of reviews of the literature on improving the frosting and defrosting performance of ASHP units, this paper provides a comprehensiv...

The impact of different parameter estimation results on the design length of a borehole heat exchanger has received very little attention. This paper provides an in-depth investigation of this problem, together with a full presentation of six data interpretation models and a comprehensive comparison of four representative sizing methods and their inter models. Six heat transfer models were employed to interpret the same thermal response test data set. It was found that the estimated parameters varied with the data interpretation model. The relative difference in borehole thermal resistance reached 34.4%, and this value was 11.9% for soil thermal conductivity. The resulting parameter estimation results were used to simulate mean fluid temperature for a single borehole and then to determine the borehole length for a large bore field. The variations in these two correlated parameters caused about 15% and 5% relative difference in mean fluid temperature in the beginning and at the end of the simulation period, respectively. For computing the borehole design length, software-based methods were more sensitive to the influence of parameter estimation results than simple equation-based methods. It is expected that these comparisons will be beneficial to anyone involved in the design of ground-coupled heat pump systems.

In domestic kitchen, cooking leads to heat generation and particle diffusion. Due to heterogeneous air mixing in partitioned spaces, difference of indoor environment between kitchen and adjacent room exits. However, study on indoor environment in adjacent room caused by cooking is not enough. Natural make-up air from exterior window openings or cracks are still the current widely used in Chinese-style residential kitchens (CRK). This study aims to predict airflow and particle distribution in a partitioned indoor environment with a CRK. Natural make-up air from exterior kitchen window, including disorganized, air infiltration and organized airflow were defined. Computational fluid dynamics method was applied to investigate velocity, air temperature, particle concentration and airflow in adjacent room. Results show that overall indoor velocity magnitude under insufficient make-up air from exterior window was two times than that under enough make-up air. Velocity magnitude in vertical and horizontal direction was quite different, but there was little difference in different position in breathing zone. Temperature difference in adjacent room ranged from 0.5 to 2.2 °C. Up to 31.8 °C was observed in disorganized make-up air in summer condition, which could lead to thermal discomfort. PM2.5 mass concentration in adjacent room could be largely reduced by organized make-up air from kitchen window. Under air volume of 810 m3/h in the model, simulated PM2.5 mass concentration in indoor environment was the lowest. With appropriate organized make-up air volume, concentration could be largely reduced. But excessive make-up air could lead to more particle diffusion from kitchen to adjacent room. Furthermore, airflow from living room to kitchen indicated a possible transmission of contaminated pollutant from dry floor drain to the living environment.

Thermal active building system (TABS) is promising to increase the flexibility of building energy systems. In this research, the thermal characteristics of a ventilated mortar block in the operation mode that the air is directly exhausted are analyzed by models. The block is integrated with phase change material (PCM) and ventilation tubes to increase both heat storage capacity and heat extraction flexibility. The simplified thermal model of the block, which combines the thermal network model and number of transfer unit model (NTU), is established and rearranged in standard state-space form based on the experimental system. The calculation process is also improved. Different modeling methods are compared, and the accuracy of the model is also validated by the testing data of three kinds of ventilated mortar blocks, which have different layers of PCM inside. Furthermore, parametric analysis is also conducted. The results show that for the current operational mode where the air is directly exhausted, the thickness of the mortar block and the diameter of the tube have a dominant impact on the surface and outlet air temperature, while velocity has a lower impact on the outlet air temperature.

Plate-fin heat exchangers (PFHE) are probably the most common type of heat exchangers. However, studies on the heat transfer performance and pressure drop of hydrocarbon mixture refrigerant in a PFHE have rarely been conducted. In this paper, boiling heat transfer and friction pressure drop characteristics of hydrocarbon mixture refrigerant in PFHE were investigated numerically. A model was established on boiling flow in vertical rectangular minichannel, and also validated by the experiment data from literature. Results indicated that the boiling heat transfer coefficient and pressure drop increased with the increase of quality and mass flux. However, they were slightly impacted by the heat flux. This was because that the main boiling mechanism was forced convective boiling, while the effect of nucleation boiling is slight on the heat transfer. The simulation data were compared with some well-known heat transfer and pressure drop correlations. The Liu and Winterton’s correlation showed the best agreement with a mean absolute deviation mostly less than ±15% for heat transfer. The calculation on Mishima and Hibiki’s correlation was less than the simulated results because that the influence of heat flux was ignored for friction pressure drop in that. Meanwhile, a new correlation for pressure drop was developed with deviation less than ±15%. The presented research is helpful in designing more effective PFHE.

Currently, icing occurs in many industries, especially in aircraft, train and grid. Ice accumulating on the equipments may result in degraded reliability, even personal injury. Therefore, deicing need to be adopted to ensure the safety operation. Nowadays, the most widely used standard deicing method is chemical deicing due to its high efficiency, although it has negative impact on environment and high cost. In addition, hot air used to deice is another potential way due to its environment friendly and effectively preventing equipment from icing up again. However, high deicing energy loss and low deicing efficiency obstruct the universality of hot air deicing. Therefore, it is necessary to experimentally study the influence factors to popularize hot air deicing. In this paper, an experimental setup was firstly built up, and then the deicing physical processes were described. This was followed by reporting the experimental results. Finally, detailed discussion and evaluation on energy consumption during deicing operation were presented. The experimental and evaluation results indicated that the favorable air temperature and velocity were 40.0–50.0 °C and 8.0–14.0 m/s, respectively, and the deicing energy efficiencies were as low as 12.1% and 8.8% when with and without recovering the waste energy.

In this paper, a saving to investment ratio (SIR) method was used to determine the key prescriptive parameters for upgrading the building energy code with different energy reduction ratio requirements, including the U value of walls, roofs and windows; as well as a consideration of the efficiency of boilers and coefficient of performance of water chillers. An economic model for single typical energy saving technologies was established and the relationship between the incremental cost and the payback period of the different technologies was analysed; then the baseline building model was optimized by these measures in descending order according to the SIR methodology. The model method (SIR) was then applied to case studies in four climate regions of China. A large office reference building situated in Harbin, Beijing, Shanghai and Guangzhou in climate zones from north to south of China was analysed as examples to showcase the relationship between the building energy saving ratio and the incremental cost. The key prescriptive parameter requirements of walls, roofs, windows, chillers and boilers with 5% and 10% energy saving ratios were defined separately for these buildings.

In order to understand the phenomenon of saturated boiling heat transfer in a vertical rectangular minichannel of the plate-fin heat exchanger (PFHE), the saturated boiling mechanism was investigated by ANSYS CFX based on the model which was validated by the experimental data from literature and the heat transfer correlation by Liu & Winterton's model was improved in terms of the experimental data from literature. The results showed that the deviation between simulation results and experiments is within ±15%. According to the simulation results, the main boiling mechanism is forced convective boiling for saturated boiling heat transfer at high vapor quality while the contribution of nucleate boiling is slight. At the same time, the evaporation of dispersed liquid phase should make the essential contribution to heat transfer. The improved correlation in terms of the experimental data from literature is in good agreement with the simulation results. The deviation between correlation calculations and simulation results is less than ±30%. These results will provide some constructive instructions for the understanding of saturated boiling phenomenon in a vertical rectangular minichannel, and for the design and prediction of heat transfer performance in PFHE.

In order to ensure the structural safety of plate-fin structures, the stress characteristics of plate-fin structures in the heat-up process of LNG plate-fin heat exchanger (PFHE) was investigated by a model which has been validated in the literature. The results show that the stresses of plate-fin structures are complex and reach the peak value in the brazed joint. A crack and fatigue failure would firstly occur in the area of the brazed joint near both the plate side and the fin side. The peak value of equivalent stress in plate-fin structures steadily increases with the increase of temperature difference between natural gas (NG) and mixture refrigerant (MR), decreases with the increase of NG pressure and it is slightly impacted by the temperature rise rate as the temperature rise rate of NG is the same as that of MR in the heat-up process. These results will provide some constructive instructions in safe operation and improvement of the heat-up process for PFHE in a large-scale LNG cold-box.

A hybrid ground source heat pump system is utilized to meet the energy demands of the first nearly-zero energy office building located in the cold region of China. The simulation result shows that the heating, cooling and lighting demands of the building could be reduced to 25 kWh/m2 yr with the utilization of high-performance envelopes and the hybrid renewable energy system. A ground loop consisting of two rectangular fields of 70 borehole heat exchangers is utilized together with the solar energy system to meet the building's energy demand. During the system commissioning phase, from 1 July to 15 August 2014, the inlet and outlet water temperatures of the heat pump unit and boreholes were collected and analysed. The coefficient of performance (COP) of the heat pump unit could reach 5.0 in cooling operation strategy, which satisfies the energy reduction requirement and meets the room temperature standard.

Adding an energy accumulator to an air source heat pump (ASHP) unit can significantly improve its defrosting performances. However, the added energy accumulator may impact the system performances during heating period, which was rarely investigated in the published studies, especially for multi-split ASHP units (a kind of more and more widely used ASHP unit). In this paper, a comparative study on system performances of a multi-split ASHP unit under frosting condition with different energy accumulator structures and energy storage methods was experimentally carried out. The experimental results showed that the multi-ASHP unit employing fin-tube energy accumulator with serial energy storage method can achieve the longest heating period, which was 3.3%, 13.4%, and 9.4% longer than that employing spiral-tube energy accumulator with serial energy storage method, spiral-tube energy accumulator with parallel energy storage method, and fin-tube energy accumulator with parallel energy storage method, respectively. For spiral-tube energy accumulator using serial energy storage method, using parallel energy storage method, can shorten the energy storage period from 59 to 33 min. For fin-tube energy accumulator using serial energy storage method, using parallel energy storage method can shorten the energy storage period from 33 to 6 min. In addition, compared to serial energy storage method, using parallel energy storage method may significantly reduce the heating capacity, input power and heating COP of the multi-split ASHP unit during the energy storage period.

This article is devoted to a review of one of the most effective systems among passive heating systems - the Trombe Wall. The main objective of this study is to revise the current potential of the Trombe Wall for cold climates. This article discusses the main subspecies of the Trombe Wall. A qualitative assessment of the cold climatic conditions use possibility was given for each subspecies. There were analyzed the most interesting studies on each structural element of the Trombe Wall, the possibility of using the Trombe Wall in high-rise buildings. The Trombe Wall effect on the exterior of the whole building was evaluated. A brief technical and economic assessment of the Trombe Wall in cold climatic conditions was carried out to determine the economic potential in this work. Based on the results of the review, the main gaps and problems of the Trombe Wall in cold climatic conditions using were identified, and the most appropriate constructive solution for this was put forward. The main directions for future studies of the Trombe Wall were established such areas as improving the Trombe Wall thermal protection, the influence of the adjustable heat transfer coefficient on the Trombe Wall thermal efficiency, developing a mathematical model that takes into account the combined operation of the Trombe Wall with the central heating source of heat.

When used in cold environment, the conventional flat plate solar collectors and all-glass evacuated tube solar collectors will cause various problems, such as large heat loss, low efficiency, freeze and tube-burst, which severely limit their applications. In this paper, a special small-sized parabolic trough solar collector (PTC), which could overcome the shortcomings of conventional solar collectors, was proposed for water heating in cold areas. An experimental platform was developed, and extensive tests were conducted to evaluate the characteristics of the proposed PTC. It was found that the thermal efficiency of the proposed PTC reached about 67% even under the condition of solar radiation of less than 310 W/m 2 , indicating that the PTC could collect solar radiation efficiently. The results also showed that when the fluid temperature was under 100 °C, the thermal efficiency was improved with increased fluid temperature, due to the increase of Reynolds number caused by the great variation of thermophysical parameters of the heat transfer fluid (HTF), which resulted from the increasing fluid temperature. Both the frosting on the reflector and the ambient wind velocity weakened the thermal efficiency greatly. The highest thermal efficiencies were respectively about 26% and 56% under condition of frosting and wind velocity of 5.0 m/s. The comparison test showed the coated double glazing receiver had a longer temperature response delay to the deflected sunlight than that of the uncoated one. The tested PTC system operated normally in low fluid temperature condition, showing great anti-freezing property in cold environment.

Sewage source heat pump was a promising technology in energy saving. To solve the fouling accumulation on sewage heat exchangers, a novel sewage-resource-based system with sewage tower equipped with guide plates was proposed to realize heat recovery and water purification. Circulating air extracted heat from sewage in the core component of sewage tower through evaporating some of the sewage, and released heat and formed condensate on evaporator surface of heat pump. Results indicated that the heat transfer rate in condenser improved from 2.629 kW to 4.876 kW and the coefficient of performance of heat pump unit ( COP u ) promoted from 3.02 to 5.01 with sewage temperature increased from 9.44 °C to 31.41 °C. COP u and the coefficient of performance of system ( COP s ) initially increased sharply and then decreased slightly with increase of the volume flow ratio of air to sewage in the sewage tower, and the turning point occurred near the volume flow ratio of air to sewage of 359:1. In addition, compared with sewage tower with baffle structure, the novel sewage heat pump system with guide plates had greater heat transfer rate of 5.71 kW at 1000 m3/h and lower air-side pressure drop in sewage tower. Meanwhile, ammonia-nitrogen concentration in the condensate was 14.72 mg/L which was 63.64% lower than that in the original sewage, indicating the water quality of condensate satisfied the requirements for urban greening and construction articulated in the water quality standard.

Although a large number of studies have been carried out on investigating the frosting behaviors and its impacts on finned tube heat exchangers (FTHXs), comparative studies for frosting behaviors on FTHXs among using different fan control modes were still lacking. Hence, a related experimental study comparing two different outdoor air fan control modes has been carried out, and the study results are presented in this paper. The study results demonstrated that compared to the constant outdoor air fan speed control mode, the constant air flow rate control mode can make the frost at the exit from leeward fins more compact and uniform, and lead to an accelerated frost growth on the surfaces of fins and tubes but an impeded frost growth on the edge of windward fins. At the end of a 60-min frosting duration, the protruded thickness of frost on the edge of windward fins, and frost thickness at the entrance to the windward fins under the constant air flow rate control mode were 7.9% and 11.1% smaller than those under the constant outdoor air fan speed control mode. In contrast, frost mass on the FTHX and the averaged frost density on the surfaces of fins and tubes under constant air flow rate control mode were 6.7% and 16.7% larger than those under constant outdoor air fan speed control mode. Furthermore, the averaged air side pressure drop, air velocity and total heat transfer rate of the FTHX under the constant air flow rate control mode were 134.5%, 82.4% and 13.4% larger than those under the constant outdoor air fan speed control mode during the 60-min frosting duration.

Since various optical factors, including sunshape and optical errors, coexist in practice, their coupling effects on the PTC’s optical performance deserve in-depth explorations. Previous studies mainly focused on individual effects of several typical optical errors or simple description of optical errors using a unified Gaussian model. Thus, this study is committed to investigating the coupling effects of multiple optical factors on the PTC’s optical performance based on the theoretically individual characterization of each optical factor. The Monte Carlo Rays Tracing method was adopted, and the effective sunshape model was established for sampling of incident rays by convolving the incident sunshape model with the specularity error model. It is revealed that larger circumsolar ratio and specularity error produced more uniform heat flux distribution on the absorber. The advantage of high optical quality reflectors in improving optical efficiency was more outstanding in clearer weather. As circumsolar ratio was more than 0.2, improving specular quality to very high degree (