Heat transfer with flow and phase change in an evaporator of miniature flat plate capillary pumped loop

An overall two-dimensional numerical model of the miniature flat plate capillary pumped loop (CPL) evaporator is developed to describe the liquid and vapor flow, heat transfer and phase change in the porous wick structure, liquid flow and heat transfer in the compensation cavity and heat transfer in the vapor grooves and metallic wall. The entire evaporator is solved with SIMPLE algorithm as a conjugate problem. The effect of heat conduction of metallic side wall on the performance of miniature flat plate CPL evaporator is analyzed, and side wall effect heat transfer limit is introduced to estimate the performance of evaporator. The shape and location of vapor-liquid interface inside the wick are calculated and the influences of applied heat flux, liquid subcooling, wick material and metallic wall material on the evaporator performance are investigated in detail. The numerical results obtained are useful for the miniature flat plate evaporator performance optimization and design of CPL.     

Experimental Study on Pool Boiling Heat Transfer for R22, R407c,and R410a on a Horizontal Tube Bundle With Enhanced Tubes

An experimental test rig for study of the pooling-boiling heat transfer performance of pure and mixed refrigerants was designed and established. The test section is a horizontal tube bundle evaporator with nine mechanically fabricated porous surface tubes in a triangular layout. With this test system, the heat transfer coefficients of the nucleate boiling in the evaporator were measured for R22, R407c, and R410a. Extensive experimental measures were made for those pure and mixed refrigerants at different heat fluxes from 10 kW m^?2 to 43 kW m^?2 at saturation temperature of 9°C. Comprehensive measured data are presented in this paper. From experimental results, it is found that the pool boiling heat transfer coefficient increases with increasing the heat flux. It is also found that boiling heat transfer coefficients for R410a are 1.25–1.81 times and 6.33–7.02 times higher than that for R22 and R407c, respectively. The experimental correlations for the pool boiling heat transfer coefficients of R22, R407c, and R410a on the present enhanced tubes bundle are developed. The thermal resistance analysis reveals that the thermal resistance of the water side is a controlling factor for the evaporator for R22 and R410a. However, for R407c, the thermal resistance of the refrigerant side is slightly higher than that of the water side. To further improve the overall heat transfer coefficient in the evaporator of R22 and R410a, the enhancement for both the inside and outside is equally important, and the effectively enhanced boiling surface must be developed for the evaporator of R407c.     

Heat transfer,flow regime and instability of a nano- and micro-porous structure evaporator in a two-phase thermosyphon loop

Two-phase flow instabilities which may occur at low and high heat loads were studied for a thermosyphon loop with R134a as refrigerant. The heat transfer surface of the evaporator was enhanced with a copper nano- and micro-porous structure. The heat transfer of the enhanced evaporator was compared to a smooth surface evaporator. Finally, the influence of the liquid level and the inside diameter of the riser on the instability of the system have been investigated.It was found that the enhanced structure surface decreased the oscillations at the entire range of heat fluxes and enhanced the heat transfer coefficient. Three flow regimes were observed: Bubbly flow with nucleate boiling heat transfer mechanism, confined bubbly/churn flow with backflow and finally churn flow at high heat fluxes.     

Thermal analysis of plate condensers in presence of flow maldistribution

Flow maldistribution in plate heat exchangers causes deterioration of both thermal and hydraulic performance. The situation becomes more complicated for two-phase flows during condensation where uneven distribution of the liquid to the channels reduces heat transfer due to high liquid flooding. The present study evaluates the thermal performance of falling film plate condensers with flow maldistribution from port to channel considering the heat transfer coefficient inside the channels as a function of channel flow rate. A generalized mathematical model has been developed to investigate the effect of maldistribution on the thermal performance as well as the exit quality of vapor. A wide range of parametric study is presented, which shows the effects of the mass flow rate ratio of cold fluid and two-phase fluid, flow configuration, number of channels and correlation for the heat transfer coefficient. The analysis presented here also suggests an improved method for heat transfer data analysis for plate condensers.     

Evaporative heat transfer model of a loop heat pipe with bidisperse wick structure

A mathematical model of evaporative heat transfer in a loop heat pipe was developed and compared with experiments. The steady-state thermal performance was predicted for different sintered nickel wicks, including monoporous and bidisperse structures. The effect of wick pore size distribution on heat transfer was taken into consideration. The wick in the evaporator was assumed to possess three regions during vaporization from an applied heat load: a vapor blanket, a two-phase region, and a saturated liquid region. The evaporator wall temperature and the total thermal resistance at different heat loads were predicted using ammonia as the working fluid. The predictions showed distinct heat transfer characteristics and higher performance for the bidisperse wick in contrast with monoporous wick. A bidisperse wick was able to decrease the thickness of the vapor blanket region, which presents a thermal resistance and causes lower heat transfer capacity of the evaporator. Additionally, a validation test presented good agreement with the experiments.     

Evaporation Heat Transfer Coefficient in a Capillary Pumped Loop and Loop Heat Pipe for Different Working Fluids

Capillary pumped loop (CPL) and loop heat pipe (LHP) are passive two-phase heat transport devices. They have been gaining importance as a part of the thermal control system of spacecraft. The evaporation heat transfer coefficient at the tooth–wick interface of an LHP or CPL has a significant impact on the evaporator temperature. It is also the main parameter in sizing of a CPL or LHP. Experimentally determined evaporation heat transfer coefficients from a three-port CPL with tubular axially grooved (TAG) evaporator and a TAG LHP with acetone, R-134A, and ammonia as working fluids are presented in this paper. The influences of working fluid, hydrodynamic blocks in the core, evaporator configuration (LHP or CPL), and adverse elevation (evaporator above condenser) on the heat transfer coefficient are presented.     

Enhancing the heat transfer performance of triangular-pitch shell-and-tube evaporators using an interior spray technique

In spray type evaporators using a conventional overhead spray method, a dry-out phenomenon occurs on the lower surface of the evaporator tubes under high surface heat flux conditions, and thus the heat transfer performance of the evaporator system is seriously impaired. This study shows that in a compact triangular-pitch shell-and-tube evaporator, the dry-out problem can be delayed through the use of an interior spray method, in which each heater tube within the bundle is sprayed simultaneously by two nozzles. The experimental results reveal that the shell-side heat transfer coefficients obtained using the proposed spray technique are significantly higher than those achieved in a conventional flooded type evaporator. The results also show that the heat transfer performance improves as the saturation temperature decreases since the density and thermal conductivity of the sprayed liquid increase. Finally, it is shown that for a constant heat flux and saturation temperature, the heat transfer coefficient increases with an increasing refrigerant mass flow rate.     

Simulation of an integrated hybrid desiccant vapor-compression cooling system

The performance of a desiccant, integrated, hybrid, vapor-compression cooling system is modeled numerically. The concept of hybrid cooling investigated in this paper utilizes the waste heat rejected from a vapor-compression cycle to activate a desiccant dehumidification cycle. The hybrid system consists of 4 major components: a compressor, an evaporator and 2 desiccant, integrated condensers/dehumidifiers. The equations governing the transport of heat and mass in the desiccant, integrated condenser/dehumidifiers are formulated considering air as the working fluid in the process stream and a refrigerant stream, which is cooled from superheated vapor to subcooled liquid, as the heat source during desorption; a water stream is used to remove the heat generated during adsorption. The governing equations are nondimensionalized and solved for both sorption processes using an explicit finite-difference scheme. The performance of a first generation prototype desiccant, integrated, hybrid, vapor-compression cooling system is then evaluated at ARI conditions.     

Thermal performance of an open thermosyphon using nanofluids for high-temperature evacuated tubular solar collectors: Part 1: Indoor experiment

An especial open thermosyphon device used in high-temperature evacuated tubular solar collectors was designed. The indoor experimental research was carried out to investigate the thermal performance of the open thermosyphon using respectively the deionized water and water-based CuO nanofluids as the working liquid. Effects of filling rate, kind of the base fluid, nanoparticle mass concentration and the operating temperature on the evaporating heat transfer characteristics in the open thermosyphon were investigated and discussed. Experiment results show the optimal filling ratio to the evaporator is 60% and the thermal performance of the open thermosyphon increase generally with the increase of the operating temperature. Substituting water-based CuO nanofluids for water as the working fluid can significantly enhance the thermal performance of the evaporator and evaporating heat transfer coefficients may increase by about 30% compared with those of deionized water. The CuO nanoparticles mass concentration has remarkable influence on the heat transfer coefficient in the evaporation section and the mass concentration of 1.2% corresponds to the optimal heat transfer enhancement.     

Determination of heat transfer coefficients for hydrogen condensation in condensers of various geometries

An important task for the hydrogen isotopes separation by cryogenic distillation is to establish the shape and dimension of the column condenser and boiler in order to obtain the desired load and separation for the distillation column. In the paper we present the set-up and experimental values for the heat transfer coefficient on various types of condensers. The heat transfer coefficients were determined by measurements on liquid hydrogen flow-rate condensed on the cold surface and temperature drop between the cooling liquid and the condensate. The experiments were made for different vapor pressures and certain temperatures of the cooling liquid from the condenser. As results we determined the condensation heat transfer coefficients for different shapes and geometries of the condensers as a function of the condensate film temperature drop.     

Heat transfer characteristics of a two-phase closed thermosyphon to the fill charge ratio

In this study, the heat transfer characteristics of a two-phase closed thermosyphon were investigated. For the test, a two-phase closed thermosyphon (copper container, FC-72 (C₆F₁₄) working fluid) was fabricated with a reservoir which could change the fill charge ratio. The experiments were performed in the range of 50-600 W heat flow rate and 10-70% fill charge ratio. Some findings are as follows.The heat transfer coefficients of the evaporator to the fill charge ratio were nearly negligible. These presented about 1-5 kW/m² K with the increase of heat flux and compared with those of smooth surface, showed some enhancement by the grooved surface. However at the condenser, the heat transfer coefficients showed some enhancement with the increase of fill charge ratio by the expanded working fluid pool. And the heat transport limitations appeared in different ways to the fill charge ratio. For the relatively small fill charge ratio (Ψ<20%), it presented about 100 W (Ψ: 10%) by the dry-out limitation.For the large fill charge ratio, it occurred by the flooding limitation and the maximum heat flow rate was about 500-550 W (Bo: 26-28), 230 W (Bo: 18.3) respectively and the Kutateladze number was about 1.9-2.1.     

不同加热方式下环路热管蒸发器可视化研究

针对环路热管内部工质相变及流动换热问题,设计了环路热管蒸发器中心通道可视化实验平台,研究了不同加热方式对热管内工质状态和传热特性的影响。结果表明：加热方式直接影响热管10W启动过程,双面加热启动速度最快。相同热载荷时,不同加热方式下环路热管热阻及蒸发器中心通道内液面高度和成核情况存在差异。10W - 40W热载荷时,随着热载荷的增大,三种加热方式的传热热阻均在减小。40W-50W热载荷时,顶部加热方式下的热管性能出现恶化,底部加热传热性能出现停滞,仅双面加热性能稳定并有提高趋势。随着热负荷的增加,蒸发器中心通道内气液界面升高、气泡的产生变得更加剧烈,蒸发器通过吸液芯向储液器的漏热量增加,进而影响环路热管的性能。     

Forced convective mixing in a zero boil-off cryogenic storage tank

This paper presents the results of a study of fluid flow and heat transfer of liquid hydrogen in a cryogenic storage tank with a heat pipe and an array of pump-nozzle units. A forced flow is directed onto the evaporator section of the heat pipe to prevent the liquid from boiling off when heat leaks through the tank wall insulation from the surroundings. An axisymmetric computational model was developed for the simulation of convective heat transfer in the system. Steady-state velocity and temperature fields were solved from this model by using the finite element method. Forty five configurations of geometry and velocity were considered. As the nozzle fluid speed increases, the values of the maximum, average, and spatial standard deviation of the temperature field decrease nonlinearly. Parametric analysis indicates that overall thermal performance of the system can be significantly improved by reducing the gap between the nozzle and the heat pipe, while maintaining the same fluid speed exiting the nozzle. It is also indicated that increased inlet tube length of the pump-nozzle unit results in slightly better thermal performance. Increased heat pipe length also improves thermal performance but only for low fluid speed.     

Investigation on the effect of filling ratio on the steady-state heat transfer performance of a vertical two-phase closed thermosyphon

Filling ratio of the working fluid has a predominant effect on the heat transfer characteristics of a two-phase closed thermosyphon (TPCT). A comprehensive model is developed to investigate the effect of filling ratio on the steady-state heat transfer performance of a vertical TPCT. Three types of flow pattern and two types of transition, according to the distribution of liquid film and liquid pool, are considered in this model, while other models generally focus on only one or two types of them. The total heat transfer rate of liquid pool, including those of natural convection and nucleate boiling, is calculated by combination of their effective areas and heat transfer coefficients. New correlations of the effective area are proposed based on the experimental results from other study. Two different geometries of the TPCT with nitrogen as working fluid are performed experimentally, and the evaporator temperatures accord well with the theoretical calculation. And the calculated results are compared with those by other empirical heat transfer correlations for liquid pool. The range of filling ratio, which can keep a TPCT steady and effective, is proposed based on analysis and comparison. The effects of heat input, operating pressure and geometries of the TPCT on the range are also discussed.     

Multi-artery,heat-pipe spreader

Multiple, columnar liquid vapor chamber allows for effective heat removal from finite, concentrated heat source by heat spreading via lateral vapor flow, while minimizing conduction resistance through thinner evaporator wick. The individual liquid arteries are designed by wick coated solid pillar. We optimize the artery geometry, numbers, and distribution, for both liquid and air-cooled, finned condensers, and show that the overall thermal resistance is substantially lower than the uniform wick vapor chamber.     

Heat Transfer Studies of a Heat Pipe

The present investigation reports a theoretical and experimental study of a wire screen heat pipe, the evaporator section of which is subjected to forced convective heating and the condenser section to natural convective cooling in air. The theoretical study deals with the development of an analytical model based on thermal resistance network approach. The model computes thermal resistances at the external surface of the evaporator and condenser as well as inside the heat pipe. A test rig has been developed to evaluate the thermal performance of the heat pipe. The effects of operating parameters (i.e., tilt angle of the heat pipe and heating fluid inlet temperature at the evaporator) have been experimentally studied. Experimental results have been used to compare the analytical model. The heat transfer coefficients predicted by the model at the external surface of the evaporator and condenser are reasonably in agreement with experimental results.     

An experimental study of steady-state behavior of a two-phase natural circulation loop

An experimental study on the steady-state behavior for a square, two-phase, natural circulation loop with water-steam as the working fluid is carried out in this study. Measurements of temperature and pressure distributions of the fluid around the loop were made under various heating and liquid-charge level conditions. Results show that fluid temperature and pressure at each location in the loop increase with increasing input power at a fixed percentage charge level. Higher fluid temperatures are observed at low percentage charge levels, and the overall heat transfer coefficient of the loop decreases with increasing percentage charge level at a fixed input power.     

Transient behavior of a radiative distiller/condenser system

In this work, a mathematical model is proposed to describe the thermal performance of a radiative distiller under transient conditions. The parameters which cause the dynamic variation in the condenser performance are the finite thermal capacity of the radiative condenser panel, effective sky-temperature, ambient temperature, humidity ratio and the condensers overall heat transfer coefficient. The presented model is solved numerically and the effects of the design and operating conditions on the condensers performance are investigated.     

Passive downward heat transport: experimental results of a technical unit

A bench scale model for passive downward transport of heat has been built and tested. The heat is transported by evaporation of a fluid in an evaporator at a higher level and condensation at a lower level. The condensate is returned to the evaporator by the periodic operation of a self-actuated float valve without disconnectding the heat delivery to the evaporator. The cost of lifting the liquid back to the evaporator is a temperature difference of a few degrees centigrade between the evaporator and the heat store. The unit works under moderate pressure (a maximum of 9 bar at 85°C). The actual vertical distance between heat source and heat store is about 1 m; a pressure difference equivalent to a vertical distance of 15 m was introduced by two spring-loaded check valves; in these conditions the ΔT between heat source and heat store is about 10°C. This ΔT is comparable to that occurring in the condenser to promote heat transfer to the heat store medium. Most of the components of the 1000 W model are similar to those used in conventional refrigerators or heat pumps. The system can easily be integrated with a solar heat collector working with a 2-phase fluid.     

A Mathematical Model of an Oscillating Heat Pipe

A mathematical model predicting the oscillating motion in an oscillating heat pipe is developed. The model considers the system multidegree oscillation of vapor bubbles and liquid plugs, including the effects of filling ratio, operating temperature, gravitational force, and temperature difference between the evaporator and condenser. The model shows that the average velocity of liquid slugs is determined by the temperature difference between the evaporator and condenser. As the turn number increases, the temperature difference for the system to start the oscillating motion decreases. Increasing the bubble number will make the system more unstable and the system can be easily started up. The existence of gravity at the bottom heating mode will make the system easily produce the oscillating motion and decrease the temperature difference as well. Results presented here will assist in optimizing the heat transfer performance and provide a better understanding of heat transfer mechanisms occurring in the oscillating heat pipe.