Heat transfer and performance characteristics of axial cooling fans with downstream guide vanes

This study examines experimentally the effect of stators on the performance and heat transfer characteristics of small axial cooling fans. A single fan impeller, followed by nine stator blades in the case of a complete stage, was used for all the experimental configurations. Performance measurements were carried out in a constant speed stage performance test rig while the transient liquid crystal technique was used for the heat transfer measurements. Full surface heat transfer coefficient distributions were obtained by recording the temperature history of liquid crystals on a target plate. The experimental data indicated that the results are highly affected by the flow conditions at the fan outlet. Stators can be beneficial in terms of pressure drop and efficiency, and thus more economical operation, as well as, in the local heat transfer distribution at the wake of the stator blades if the fan is installed very close to the cooling object. However, as the separation distance increases, enhanced heat transfer rate in the order of 25% is observed in the case of the fan impeller.     

Water–zeolite adsorption heat pump combined with single effect evaporation desalination process

The single effect evaporation desalination process combined with adsorption heat pump (ADVC) is modeled analyzed as a function of the system design and operating parameters. The analysis gives variations in the thermal performance ratio, the specific heat transfer area, and the specific flow rate of cooling water. The performance evaluation is made as a function of the brine boiling temperature, the difference in the temperature of the compressed vapor and the boiling brine, and the water content in the adsorption bed. Results show that the thermal performance ratio of this configuration is the highest among all single effect evaporation desalination systems. Moreover, the specific flow rate of the cooling water and the specific heat transfer area are similar to those of other single effect configurations. It should be stressed these promising features makes the ADVC system highly attractive to small and remote communities and of special interest in situations where energy cost is high.     

Theoretical analysis of ammonia-water absorption cycles for refrigeration and space conditioning systems

This paper presents an investigation of an ammonia-water absorption cycle for solar refrigeration, airconditioning and heat pump operations at higher heat supply temperatures. The system consists of a solar driven generator, rectifier, condenser, evaporator, absorber and heat exchangers for preheating and subcooling within the system. A steady state thermodynamic cycle analysis based on mass and heat balances along with the state equations for the thermodynamic properties of the ammonia-water mixture has been carried out. A numerical computer simulation of the system with input component temperatures, refrigerant concentration/mass flow rate and effectiveness of the heat exchangers has been made to evaluate the relative heat transfer rates (i.e. coefficients of performance) and the mass flow rates for the cooling/heating modes. It is found that unlike the low generator temperature behaviour the coefficients of performance for both cooling and heating modes are reduced at higher generator temperatures. However, an increase of condenser temperature for each mode of operation improves the performance of the systems at higher generator temperatures. A choice for keeping the absorber temperature equal to/lower than that of the condenser is also predicted at lower/higher generator temperatures, respectively. In general the results are more pronounced for the refrigeration mode than for the heat pump mode and are least effective for the airconditioning mode.     

Experimental volumetric heat transfer characteristics between oil and glass pebbles in a small glass tube storage

A small glass tube containing oil and glass pebbles is constructed to evaluate rapidly the volumetric heat transfer characteristics during charging. An electrical hot-plate in thermal contact with a steel spiral coil is used to charge the oil/pebble-bed storage under different average charging flow rates. The heat transfer characteristics are presented in terms of the average temperature of the oil, the average temperature of the pebbles, the average charging power and the volumetric heat transfer coefficient. The results reveal that an increase in the average charging flow rate results in a linear increase in the volumetric heat transfer coefficient. An expression for the correlation between the superficial mass flow velocity, the average pebble diameter and the volumetric heat transfer coefficient is also formulated.     

新型冷轧管管壳式油冷器的研制

尽管壳式油冷器已被大量应用,但冷却元件结构进一步优化对改进其传热性能仍有较大潜力。本文介绍了在二维温度场测定的基础上,优化了冷却元件结构,设计了新颖的油冷器情况,试验证明,它对进一步缩小油冷器体积,提高冷却效率有明显的效果。     

The first and second law analysis of a lithium bromide/water coil absorber

The aim of this paper is to study the irreversibilities in a coil absorber using lithium bromide solution and to determine the variation of the second law efficiency with some variables such as cooling water flow rate, solution flow rate, cooling water temperature and solution concentration. The influence of absorber performance parameters is examined on the basis of the first and second laws of thermodynamics for parallel and counter-current types. In this regard, the heat and mass transfer, the second law efficiency, the magnitude and place of exergy losses in two types of absorbers are estimated and discussed comprehensively. The results showed that increasing the cooling water flow rate and decreasing the cooling water inlet temperature increase the heat and mass transfer, and decrease the second law efficiency. The effect of the solution concentration on the efficiency in general is small. Whereas the irreversibility for the counter-current mode is greater than that of the parallel-current mode, the heat-mass transfer 3–19% and the second law efficiency 1–12% are higher.     

An overview of the effect of lubricant on the heat transfer performance on conventional refrigerants and natural refrigerant R-744

This review provides an overview of the lubricant on the heat transfer performance, including nucleate boiling, convective boiling, shell side condensation, forced convective condensation, and gas cooling, for conventional refrigerants and natural refrigerant R-744. Various parameters affecting the heat transfer coefficient subject to lubricant, such as oil concentration, heat flux, mass flux, vapor quality, geometric configuration, saturation temperature, thermodynamic and transport properties are discussed in this overview. It appears that the effect of individual parameter on theProd. Type: FTP heat transfer coefficient may be different from studies to studies. This is associated with the complex nature of lubricant and some compound effect accompanying with the heat transport process. In this review, the authors try to summarize the general trend of the lubricant on the heat transfer coefficient, and to elaborate discrepancies of some inconsistent studies. The lubricant can, increase or impair the heat transfer performance depending on the oil concentration, surface tension, surface geometry, and the like. For the condensation, it is more well accepted that the presence of lubricant normally will impair the heat transfer performance due to deposited oil film. However, the deterioration is comparatively smaller than that in nucleate/convective boiling. For the effect of lubricant on R-744 with convective evaporation, the general behavior is in line with the convectional refrigerant. For gas cooling, the lubricant cast significant effect on heat transfer coefficient especially for a higher mass flux or at a smaller diameter tube.     

Cooling Augmentation with Microchanneled Structures

Experiments were conducted to investigate the heat transfer characteristics and cooling performance of subcooled liquid, water, flowing through rectangular cross-section microchanneled structures machined on a stainless steel plate. Heat transfer or flow mode transition was observed when the heating rate or wall temperature was increased. This transition was found to be suggestively induced by the variation in liquid thermophysical properties due to the significant rise of liquid temperature in the microstructures. The influence of such parameters as liquid velocity, subcooling, property variation, and microchannel geometric configuration on the heat transfer behavior, cooling performance and the heat transfer and liquid flow mode transition were also investigated. The experiments indicated that both slngle-phase forced convection and flow boiling characteristics were quite different from those in normal-sized tubes and the heat transfer was obviously intensified.     

单回路紫铜_水脉动热管传热性能的实验研究

实验研究了单回路紫铜—水脉动热管在水冷方式和定传热功率时,冷却水流量、倾角、管径和充液率4种因素对热管传热性能,包括管壁测点温度、冷热段均温、传热温差、传热热阻和温度振幅的影响规律,得到提高传热性能的一些措施。结果显示:水平放置的单回路脉动热管无法启动;30°以上倾角管内可产生振荡,增加倾角可降低传热热阻;定加热功率下,冷却水流量存在最佳值,过大和过小都会增加传热热阻;在脉动热管允许管径范围内,增加管径可大大降低传热热阻;相同传热功率时,30%充液率热管的传热热阻明显低于70%充液率管;小而均匀的壁温振荡比大幅锯齿状振荡时的传热性能好。     

Heat pump system utilizing produced water in oil fields

As the alternative to the heating furnace for crude oil heating, a heat pump system utilizing produced water, a main byproduct, in oil fields was proposed and the thermodynamic model of the system was established. A particular compression process with inner evaporative spray water cooling was applied in the screw compressor and an analysis method for the variable-mass compression process was introduced. The simulation results showed that the efficiency of the screw compressor, the temperature of produced water and the temperature difference in flash process are key parameters affecting the system performance. The energy cost of the heat pump system was compared to that of the heating furnace, revealing that the heat pump system with EER, 4.67, would save over 20% energy cost as compared with the heating furnace. Thus, the heat pump system was energy saving, money saving and environmentally benign.     

Irradiation Target Cooling Using Circular/Slot Air Jet

To study the effect of irradiation on materials, sample coupons are irradiated in cyclotron facilities. During the irradiation process, these samples produce significant heat. This heat needs to be continuously removed from the samples in order to avoid melting of the samples as well as to keep the samples at a particular temperature during irradiation. The area available for heat transfer is limited due to small size of the samples. To increase the heat transfer rate, jet cooling is used as it provides large heat transfer co-efficient. To understand the heat transfer characteristics of jet cooling under these conditions, experiments have been carried out. Two inclined jets hitting on both sides of the target plate give maximum cooling and uniform temperature distribution. This paper gives the details of the numerical and experimental studies carried out and the discussions about the results obtained.     

Conjugate heat transfer investigation on the cooling performance of air cooled turbine blade with thermal barrier coating

A hot wind tunnel of annular cascade test rig is established for measuring temperature distribution on a real gas turbine blade surface with infrared camera. Besides, conjugate heat transfer numerical simulation is performed to obtain cooling efficiency distribution on both blade substrate surface and coating surface for comparison. The effect of thermal barrier coating on the overall cooling performance for blades is compared under varied mass flow rate of coolant, and spatial difference is also discussed. Results indicate that the cooling efficiency in the leading edge and trailing edge areas of the blade is the lowest. The cooling performance is not only influenced by the internal cooling structures layout inside the blade but also by the flow condition of the mainstream in the external cascade path. Thermal barrier effects of the coating vary at different regions of the blade surface, where higher internal cooling performance exists, more effective the thermal barrier will be, which means the thermal protection effect of coatings is remarkable in these regions. At the designed mass flow ratio condition, the cooling efficiency on the pressure side varies by 0.13 for the coating surface and substrate surface, while this value is 0.09 on the suction side.     

Experimental performance of a thermoelectric ceiling cooling panel

An experiment has been performed to investigate the cooling performance of a thermoelectric ceiling cooling panel (TE‐CCP). The TE‐CCP was composed of 36 TE modules. The cold side of the TE modules was fixed to an aluminum ceiling panel to cool a test chamber of 4.5 m³ volume, while a copper heat exchanger with circulating cooling water at the hot side of the TE modules was used for heat release. Tests were conducted using various system parameters. It was found that the cooling performance of the system depended on the electrical supply, cooling water temperature and flow rate through the heat exchanger. A suitable condition occurred at 1.5 A of current flow with a corresponding cooling capacity of 289.4 W which gives the coefficient of performance (COP) of 0.75 with an average indoor temperature of 27°C. Using thermal comfort test data in literature for small air movements under radiant cooling ceilings, results from the experiments show that thermal comfort could be obtained with the TE‐CCP system. Copyright © 2007 John Wiley & Sons, Ltd.     

Cooling performance of an impingement cooling device combined with pins

Experimental study and one dimensional model analysis were conducted to investigate cooling performance of an integrated impingement and pin fin cooling device. A typical configuration specimen was made and tested in a large scale low speed closed-looped wind tunnel. Detailed two-dimensional contour maps of the temperature and cooling effectiveness were obtained for different pressure ratios and therefore different coolant flow-rates through the tested specimen. The experimental results showed that very high cooling effectiveness can be achieved by this cooling device with relatively small amount of coolant flow. Based on the theory of transpiration cooling in porous material, a one dimensional heat transfer model was established to analyze the effect of various parameters on cooling effectiveness. It was found from this model that the variation of heat transfer on the gas side, including heat transfer coefficient and film cooling effectiveness, of the specimen created much more effect on its cooling effectiveness than that of the coolant side. The predictions of the one-dimensional mode were compared and agreed well with the experimental data.     

Predictions of In-Tube Cooling Heat Transfer Coefficients and Pressure Drops of CO2 and Lubricating Oil Mixture at Supercritical Pressures

The in-tube cooling heat transfer and flow characteristics of supercritical pressure CO₂ mixed with small amounts of lubricating oil differ from those for pure CO₂ due to the entrainment of the lubricating oil as well as the sharp property variations of the supercritical CO₂ working fluid. In-tube gas cooling flow and heat transfer models were developed in this study for CO₂ with entrained polyol ester type lubricating oil in a CO₂ gas cooler at supercritical pressures. A “thermodynamic approach,” which treats the CO₂–oil mixture as a homogenous mixture was used with the heat transfer coefficients and frictional pressure drops evaluated based on the thermophysical properties of the CO₂–oil mixture. Thermophysical property variation correction terms as a function of the wall temperature and the oil concentration were included in the models. The frictional pressure drop correlation predicts more than 90% of the experimentally measured data within ±10%, while the heat transfer coefficient correlation predicts more than 90% of the experimentally measured data within ±20%.     

冷却流量对涂层叶片冷却性能影响的数值研究

采用气热耦合数值方法研究了冷却流量对热障涂层气冷涡轮叶片冷却性能的影响,并对结果进行了对比分析。研究结果表明：热障涂层叶片的综合冷却效率随冷却流量的增加而增大,但增幅则逐渐下降。在吸力面上,附加热障涂层的效果更好。基准工况下,附加热障涂层,叶片表面温度可降低72.6 K,综合冷却效率增幅最大可达6.5%。在尾缘区域,热障涂层会阻碍热量从金属叶片表面向低温的流体传递,导致叶片表面性能下降,因此,只有配合高效的内冷技术,才能达到理想的冷却效果。     

Experimental investigations on overall cooling effect of ribbed channel with air bleeds

An experimental investigation on overall heat transfer performance of a rectangular channel, in which one wall has periodically placed oblique ribs to enhance heat exchange and cylindrical film holes to bleed cooling air, has been carried out in a hot wind tunnel at different mainstream temperatures, hot mainstream Reynolds numbers, coolant Reynolds numbers and blowing ratios. To describe the cooling effect of combined external coolant film with the internal heat convection enhanced by the ribs, the overall cooling effectiveness at the surface exposed in the mainstream with high temperature was calculated by the surface temperatures measured with an infrared thermal imaging system. The total mass flow rate of cooling air through the coolant channel was regulated by a digital mass flow rate controller, and the blowing ratio passing through the total film holes was calculated based on the measurements of another digital-type mass flow meter. The detailed distributions of overall cooling effectiveness show distinctive peaks in heat transfer levels near the film holes, remarkable inner convective heat transfer effect over entire channel surface, and visible conductive heat transfer effect through the channel wall; but only when the coolant Reynolds number is large enough, the oblique rib effect can be detected from the overall cooling effectiveness; and the oblique bleeding hole effect shows the more obvious trend with increasing blowing ratios. Based on the experimental data, the overall cooling effectiveness is correlated as the functions of Re_m (Reynolds number of hot mainstream) and Re_c (Reynolds number of internal coolant flow at entrance) for the parametric conditions examined.     

Experimental validation of CFD modelling for heat transfer coefficient predictions in axial flux permanent magnet generators

This paper describes the experimental validation of CFD modelling for heat transfer coefficients in an axial flux permanent magnet (AFPM) generator. A large scale low speed test rig was designed and constructed. The geometric parameters and the rotational speed of the test rig were determined by dimensional analysis, to ensure the flow characteristic remains unchanged as compared with commercial AFPM generators. The heat transfer coefficients in the test rig were measured at rotational Reynolds number, Re_ω from 0 to 2 × 10⁶, non-dimensional flow rate, C_w up to 11,000 and gap ratio, G = 0.016, by using the combination of heat flux sensors and thermocouples. Due to the large size of the scaled-up rig, natural convection played a significant part in the heat transfer and this had to be compensated for in the forced convection heat transfer coefficient calculations. Extra experiments were designed and conducted to identify the effect of natural convection on the machine’s cooling. The experimentally determined results were compared to heat transfer coefficients predicted by CFD models and good agreement was obtained.     

Experimental study on thermodynamic vent system with different influence factors

A ground experiment is established to investigate the pressure control performance of thermodynamic vent system (TVS) with HCFC123. Different influence factors, including tank pressure control bands, circulation volume flow rates, and heat loads, are investigated separately. The variations of tank pressure and fluid temperature are analyzed in different operation process. To compare the performance of TVS with that of direct venting, the tank heat leakage is solved with a quasi‐steady heat transfer model. With the actual penetration heat into tank determined, the performance comparison is made between direct venting and TVS. The results show that the increase rate of tank pressure rises with the heat load during the pressurization process. While the bulk fluid is still subcooled, great tank pressure control and fluid cooling could be obtained by increasing the circulation flow rate in the mixing injection process. With the cold capacity generated by the throttling process, both the vapor and liquid should be well cooled. For the case of No.3 to 4, as the refrigeration capacity of TVS could not eliminate the accumulated heat load timely, the fluid has a temperature increase in the early stage of throttling process. While for the case of No.6, with the fluid being cooled sufficiently, both the vapor and liquid have received great temperature control. Compared with the direct venting, the recovery ratio of venting gas loss generated by TVS ranges from 30% to 145%, which shows the TVS has a large advantage on exhaust saving.     

Investigation of spray cooling: Effect of different heater surfaces under acceleration

Spray cooling characteristics under acceleration conditions for different heat transfer surfaces were experimentally investigated with the aid of an acceleration test bench. Three heater surfaces tested were the flat surface, straight fin surface and porous tunnel surface. Water was used as the coolant and sprayed onto the heater surface at various nozzle heights by nozzles having different flow rates. The results of the fundamental studies show that for all the three surfaces, flow rate as well as nozzle height affects the spray cooling performance under the acceleration condition just like it does under the stationary condition. The influence of acceleration for the three heater surfaces is different and for one specific surface the effect of acceleration is also different due to the change in flow rate or nozzle height. According to the comparison results of the heat transfer performance for three surfaces under acceleration condition, it is observed that the porous tunnel surface has the best performance in all tests, followed by the flat surface in most tests. While for a certain case with a low flow rate and low nozzle height, the spray cooling capability for straight fin surface is better than flat surface.