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A passive method with phase change material (PCM) is an appropriate technique in electronic cooling. But, due to its poor thermal conductivity, many enhancers are employed to reduce the thermal resistance offered by the PCM. A partial filling strategy to reduce the cost and weight of foams with fins is used in this study. A hybrid heat sink with a combination of fins placed at the sidewalls of the enclosure and foams filled at certain heights such as 10, 20, and 30 mm is considered in this present work. A two-dimensional numerical model with n-eicosane as PCM is developed in ANSYS Fluent 19. A multi-objective optimization is carried out using a reliable multi criteria decision making approach. Different weightage is distributed to the objective functions in this method depending on the choice of the user. The pore size and density vary for various filling heights, and 60 cases are investigated for both charging and discharging cycles. The pore size of 0.8-0.95 and pore density of 5-25 pores per inch with a broad range is considered. From the discussions, guidelines for selecting a preferable pore size and pore density can be determined based on the filling height and applied weightage. [ABSTRACT FROM AUTHOR]

Air cooling systems are widely used in current data centers owing to their low capital costs and high reliability. To satisfy the increasing rack power density, the optimal air-cooling technology and an economic analysis should be carefully discussed. Therefore, this study discusses four airflow management technologies: Case 1: raised floor and cold aisle containment supply/computer room air conditioning (CRAC) direct return; Case 2: CRAC direct supply/hot aisle containment (HAC) return; Case 3: overhead duct supply/CRAC direct return; and Case 4: overhead duct supply/HAC return. Using a validated model, the thermal and economic performances of each case were compared. Results showed that Case 4 exhibited the best thermal performance, followed by Cases 3, 2, and 1. Case 1 cannot satisfy the heat dissipation requirement when the rack power density is larger than 12.5 kW; whereas only Case 4 can be used when the power density is larger than 15 kW. Regarding location within China, owing to the high ambient temperature, Shenzhen showed the highest annual cost value and power usage effectiveness, followed by Shanghai, Xi'an, Beijing, and Harbin. Finally, Cases 3 and 4 are recommended for application when the rack power density is greater than 10 kW. [ABSTRACT FROM AUTHOR]

NOx emission reduction from gas boilers has become a key issue in improving air quality. Combustion air humidification technology is gradually being used to reduce NOx emissions. However, the NOx emission reduction effect of gas boilers at a higher combustion air humidity has been studied less. A flue gas with low NOx emissions and a waste heat recovery system using combustion air humidification technology are proposed in this study. In the ultra-low NOx mode, the effect of high combustion air humidity on NOx emission reduction and efficiency of the gas boiler were studied experimentally. In the waste heat recovery mode, the effects of the heat network backwater temperature on the NOx emission reduction and system efficiency were studied experimentally. Results showed that an increase in air humidity can significantly reduce the NOx concentration formed by combustion. The ultra-low NOx mode reduces NOx emissions from 130mg/m3 to 23.3 mg/m3 and affects the boiler efficiency slightly. In the waste heat recovery mode, NOx emissions can be reduced to 39.9mg/m3 when the backwater temperature of the heat network is 55 °C. This condition improves the efficiency to 93.8%. The analysis results provide suggestions for the selection of the operation modes. [ABSTRACT FROM AUTHOR]

Insulation performance and buried depth of heating pipelines are the vital factors affecting the energy loss of directly buried heating pipelines. This study considers the thermo-economic assessment of insulation of directly buried heating pipelines for district heating. The optimal insulation thickness, energy-saving effect, and payback period of district heating pipelines with five nominal pipe diameters, four fuel types, four kinds of insulation materials, and four buried depths are calculated. A numerical code is developed based on Life Cycle Cost Analysis and is validated via comparing with results in the open literature. Three representative cities of Xi'an, Shenyang, and Harbin subjected to three different climatic zones in China are explored. The highest values of optimum insulation thickness are 176, 153, and 121 mm in Harbin, Shenyang, and Xi'an, respectively, which are reached using oil as fuel, rock wool as insulation material, nominal pipe diameter 500 mm, and buried depth 1 m. A sensitivity analysis is performed to indicate how much the optimum insulation thickness and payback period are sensitive to the changes of insulation, fuel, and buried depth. The results show that insulation and fuel have a greater influence on the optimum insulation and payback period than the buried depth. [ABSTRACT FROM AUTHOR]

In this paper, experimental data pertinent to deionized water pool boiling across 10 mm thick aluminum foams are presented. Two foam samples with different pore densities, 5 and 10 pores per inch, yet an identical mean porosity of 0.92 are tested. Compared to a heated flat plate, of the same base size, the foams offer higher heat transfer area albeit at induced bubble escaping resistance. The tradeoff between these two effects is investigated. Through the use of high-speed camera recording, bubble generation, trajectory and growth rate were analyzed and critically discussed as functions of the working conditions and of the foam geometry. Furthermore, the experimental data were compared against a correlation appositely proposed for water pool boiling on metal foams. A good degree of agreement was observed. [ABSTRACT FROM AUTHOR]

The results showed that ZnO based nanofluid improved the natural convection heat transfer at 0.10 vol.%, at higher concentrations the natural convection of nanofluid deteriorated. The eleventh paper by Wang and Okawa studied the effect of nanoparticle layer coating on heat transfer performance of heat pipe. This Special Issue of I Heat Transfer Engineering i includes thirteen selected papers from the 1 SP st sp International Conference on Nanofluids (ICNf), which was held at Universitat Jaume I, in Castelló (Spain), on 26 SP th sp to 28 SP th sp June 2019. [Extracted from the article]

Extensive computational investigations have been performed to obtain more detailed information about the peculiar phenomena of turbulent supercritical carbon dioxide (sCO2) flow as ideal heat transfer fluids in various thermal engineering applications. This paper reviews the simulation techniques used and discusses their advantages, shortcomings and applicability. Not only is a comprehensive inspection on various computational approaches provided, but also the model refinements are suggested. Direct Numerical Simulations (DNS) provides valuable and reliable information about the thermohydraulics of turbulent sCO2 flows, in particular within the near-wall region, which well interprets the observed heat transfer enhancement and deterioration with property variations, flow acceleration and buoyancy discussed. However, DNS is not feasible when it comes to high Reynolds number flows with complex geometries encountered in practical applications because of the drastically increasing computational cost. Reynolds-Averaged Navier-Stokes (RANS) modeling is able to fill the gap with acceptable accuracy and becomes the mainstream for turbulent sCO2 heat transfer simulations. The flow and heat transfer behaviors of turbulent sCO2 can be simulated using RANS modeling leading to acceptable predictions. However, the performance variation is considerable for different models and for the same model of changing operating conditions, model generality is not reached. In addition, some treatments implemented into the RANS models for constant property fluids are not appropriate for variable-property sCO2 flows, causing inconsistency on the mixed convection predictions. Variable turbulent Prandtl number and more advanced calculation schemes for buoyancy production of turbulent kinetic energy are strongly recommended. Also, more appropriate treatments for damping functions are demanded to enable the model properly respond to the local property changes, particularly near the wall. Much simpler models with far less computational cost based upon the two-layer theory are being developed to achieve the generality. While this is promising, the examinations are still limited to the certain conditions and some model parameters need to be calibrated against the DNS data, which definitely reduces the model universality since DNS only covers a limited range of operating conditions. Developing more generic and reliable RANS models is still the main focus of simulation techniques used for turbulent sCO2 heat transfer. [ABSTRACT FROM AUTHOR]

The present paper provides an overview and review on heat and mass transfer involved in evaporation of single saline water droplets as well as pure and saline water sprays for cooling purposes. The aim is to demonstrate the advances that have been made toward the deployment of Natural Draft Dry Cooling Tower operating with saline water. A new theoretical model for evaporation of single solid containing droplet is reviewed, and the corresponding advantages are discussed. Moreover, a comparison between pure and saline water sprays is reported and performance correlations are compared. The new approaches to implement saline water injections in numerical simulation and nozzle arrangement in a cooling tower reported in this work provides valuable tools for designing an efficient cooling tower. This work shows that in addition to fresh water preservation and cost savings due to water purification, using saline water can also lead to enhance cooling efficiency by up to 8% and build more compact cooling towers. Overall it has been shown that the use of saline water is a viable and promising concept that requires further exploration. [ABSTRACT FROM AUTHOR]

This paper reports an investigation into a practical cooling issue on a type of fan-forced finned-tube heat exchangers used in Queensland's coal seam gas (CSG) industry. CSG compression facilities in some production sites suffered underproduction in recent summers because of frequent automatic engine shutdowns. The problem is not expected by the manufacturer's design. However, it is suspected of being related to the control systems on the compression facilities triggering the overheating-protection shutdowns due to possible deficiencies in one or some water/gas cooling loops in the facilities’ air-cooled heat exchangers. Therefore, to understand which heat exchangers and what exact reasons cause the unexpected cooling issue, an investigation has been carried out on the cooler units of the gas compression facilities. A field instrumentation measurement on one operating cooler unit has been done, followed by an analysis using a one-dimensional analytical model and a three-dimensional computational fluid dynamics model. The experimental results are used to validate both the models. Then the cooling performance of the cooler unit under the summer peak condition is predicted by the verified models. The prediction suggests that the water inlet temperature in one particular cooler section is higher than its upper limit defined by the manufacturer, due to poor cooling at high ambient temperatures. The lower cooling performance is caused by large reductions in the cooler air speed and total heat transfer coefficient, which are related to less efficiency of the cooler fans, more airflow resistance, and fouling on both sides of the finned tubes. [ABSTRACT FROM PUBLISHER]

The aim of this study is to simplify the process of discharge thermal energy combined desalination with power system by integrating the two existing heat exchangers (condensers) into a new multistream one. This system is a heat recovery unit, which is used to cogenerate water and power. Two shell-and-tube condensers operate in a closed power cycle and a desalination system for cooling an ammonia mixture as a working fluid and condensing a pure vapor, respectively. Here, a two-phase flow multistream condenser is utilized instead of the two low-exergy-efficiency shell-and-tube condensers. The results proved that the proposed technique leads to improving its exergy efficiency by 15%. The performance of the proposed condenser was analyzed by applying parametric optimization. [ABSTRACT FROM PUBLISHER]

In recent years, open-cell metal foam has gained attention for utilization for exhaust gas recirculation coolers due to its large surface area and porous structure. Theoretically, the porous foam structure would have better transfer heat through conduction and convection processes. However, the exhaust gases that enter the cooler would carry particulate matter, which may deposit within the foam structure. The existing fouling studies cannot explain the underlying mechanisms of particulate deposition thoroughly within the foam structure. This study reviews the particulate fouling of heat exchangers, particularly in the exhaust gas recirculation system. Some past approaches to investigate fouling, particle transport, and deposition in the metal foam heat exchangers for many different applications are also included. In addition, this study also includes the challenges that lie ahead in implementing the metal foam heat exchangers in the industries. [ABSTRACT FROM PUBLISHER]

A simple theoretical model, validated against available numerical and experimental data in the literature, is presented to predict the effects of crosswind on the performance of natural draft dry cooling towers. The intersection of asymptote method, along with scale analysis, is used to find a closed-form solution for the airflow rate at the tower exit for given crosswind speeds. The total heat rejected under a windy condition is then calculated based on the air mass flow rate at the tower exit. This theoretical model allows for parametric studies and can generate accurate data. Interestingly, the model results, expected to be accurate within an order of magnitude, are more accurate than anticipated when compared to available experimental and numerical data in the literature. In fact, the maximum relative error is observed to be 15% when current theoretical predictions are compared to available experimental data. The results of this study will be useful for future work on the development of air-cooled condensers, especially for geothermal and solar thermal power plants in Australia. [ABSTRACT FROM PUBLISHER]

An introduction is presented in which the guest editor discusses topics including the use of heat exchangers in thermal power plants, developing technologies for improving performance of cooling towers, and the advantages of dry cooling.

In the present study, lattice Boltzmann simulation of conjugate heat transfer from multiple heated obstacles mounted in a walled parallel-plate channel is carried out. The aim is to investigate the effects of the pertinent thermophysical and geometrical parameters on the local Nusselt number around the obstacle's periphery. The result showed that increasing the obstacle to fluid thermal conductivity ratio and decreasing wall-to-obstacle thermal conductivity ratio results in increasing the local Nusselt number around the obstacle's periphery. The results of the present study are compared with those available from the conventional numerical methods, and good agreement is observed. [ABSTRACT FROM AUTHOR]

An energy and exergy study has been done on a double-pass flat-plate solar air collector with and without porous medium embedded inside the lower channel of the collector. Energy conservation equations are used to derive the energy-governing equations for all components of the collector. A second law analysis (the second law of thermodynamics) was carried out to optimize the mass flow rate, which leads to optimization of the energy efficiency. Theoretical results for energy and exergy efficiencies are plotted for the cases of with and without embedded porous medium. Results of this research show that the porous medium embedded inside the lower channel leads to an increase in the thermal efficiency of the collector of more than 30% compared with the case without porous medium, hence showing the importance of employing porous medium in thermal solar collectors. On the other hand, the pressure drop in the air caused by friction with porous medium is not negligible and is also studied here, using the second law analysis. [ABSTRACT FROM AUTHOR]