Heat transfer in the evaporator section of moderate-speed rotating heat pipes

Experiments were performed to investigate the heat transfer mechanism in the evaporator section of non-stepped rotating heat pipes at moderate rotational speeds of 2000–4000 rpm or accelerations of 40g–180g, and evaporator heat fluxes up to 100 kW/m². The thermal resistance of the evaporator section as well as that of the condenser section was examined by measuring the axial temperature distributions of the flow in the core region of the heat pipe and along the wall of the heat pipe. The experimental results indicated that natural convection heat transfer occurred in the liquid layer of the evaporator section under these conditions. The heat transfer measurements were in reasonable agreement with the predictions from an existing rotating heat pipe model that took into account the effect of natural convection in the evaporator section.     

Heat Transfer in a Loop Heat Pipe using Diamond-H2O Nanofluid

The main aim of this study is to enhance the thermal performance of loop heat pipe (LHP) charged with nanofluid as the working fluid. Thus, experiments are conducted to investigate heat transfer characteristics of using diamond-H₂O nanofluid with nanoparticle mass concentration ranged from 0% to 3% in a LHP as a working medium for heat input range from 20 W to 60 W. The three-dimensional model, laminar flow and heat transfer governing equations are solved using the finite volume method. The simulations are carried out with three-dimensional model based on the characterization of the working fluid inside the LHP to give an insight into the heat transfer and fluid flow mechanism. The LHP performance is evaluated in terms of temperature distributions and total thermal resistance of LHP. It is inferred that the temperatures obtained at all points in evaporator side of LHP charged with diamond-H₂O nanofluid are lower and reach their steady state faster than LHP charged with pure water. At the constant heat input, test results showed the average decrease of 5.7%?10.8% at nanoparticle mass concentrations ranging from 0.5% to 3% in R_th of LHP as compared with pure water (0%).     

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.     

Second law analysis for microscale flow and heat transfer

In phase change transport devices, capillary forces drive overall circulation of working fluid from an evaporator section to a condenser section, whereas the thin film flows at the evaporating meniscus are driven by capillary and disjoining pressure gradient. An analysis has been provided for the microscale fluid flow and heat transfer in an evaporating extended meniscus. Using the second law of thermodynamics, the entropy generated has been investigated. The geometric configuration that corresponds to the minimization of entropy generated and minimization of fluid flow resistance is identified.     

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.     

Thermal response of a heat pipe with axially “Ω”-shaped microgrooves

A transient model of capillary flow and heat transfer in a heat pipe with axially “Ω”-shaped microgrooves is developed and numerically analyzed to predict the thermal response characteristics. The transient distributions of the axial capillary radius and solid wall temperature, the evaporating mass rate, the time constant and instantaneous effective thermal conductivity are all investigated and discussed. The results indicate that the rise rate of wall temperature during the initial period is relatively larger, and the coordination of solid wall temperature response among the evaporator, adiabatic and condenser section is realized during the whole startup process. When the input power is increased/decreased, the evaporator temperatures start rising/dropping immediately. In particular, the time constant and instantaneous effective thermal conductivity in the startup process are larger than those in the shutdown process. Additionally, the accuracy of the present model is verified by experimental data obtained in this paper.     

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。     

Hotspot Analysis of Double-Layer Microchannel Heat Sinks

ABSTRACT

The performance of double-layer microchannel heat sinks are evaluated comparatively for the parallel flow, counter flow, and transverse flow configurations with and without hotspots as heating condition. Conjugate heat transfer analysis is performed by solving three-dimensional Navier–Stokes and energy equations using a finite volume solver. The flow is considered to be steady, incompressible, and laminar. Functional relations between the thermophysical properties of water and temperature are developed and used for numerical calculations with variable fluid properties. The thermal resistances, maximum temperature increase at the hotspots, temperature variation among the hotspots, and pressure drops are evaluated for the three heat-sink designs with two hotspot schemes (single hotspot at the center of the heat sink and multiple hotspots distributed uniformly at six peripheral locations). For the single-hotspot case, the parallel flow heat sink exhibited the lowest thermal resistance and temperature rise at the hotspot. For all the six multiple hotspot cases, the transverse flow heat sink exhibited the lowest thermal resistance and temperature variation among the hotspots.     

Evaporation and condensation heat transfer in a heat pipe with a sintered-grooved composite wick

A mathematical model of evaporation and condensation heat transfer in a copper-water wicked heat pipe with a sintered-grooved composite wick is developed and compared with experiments. The wall temperatures are measured under different input power levels and working temperature conditions. The results show that the heat transfer in the condenser section was found to be only by conduction. In the evaporator, however, either conduction or boiling heat transfer can occur. The experimental data for the boiling heat transfer are well correlated by the theory of Stralen and Cole. Higher heat load drives the heat pipe to spend more time achieving the equilibrium state during the transient start-up process. The response curves of the evaporator thermal resistance are overlapped, and the condenser thermal resistance increases more sharply at the beginning. The total thermal resistance of the heat pipe ranges from 0.02 to 0.56 K/W.     

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.