Evaporative Cooling Heat Transfer of Water From Hierarchically Porous Aluminum Coating

A study of evaporative cooling of water was conducted using dual-scale hierarchically porous aluminum coating. The coating was created by brazing aluminum powders to a flat aluminum plate. The effects of particle size and thickness on evaporative heat transfer were investigated using average aluminum particle diameters of 27, 70, and 114 µm and average coating thicknesses of 560, 720, and 1200 µm. Constant ambient temperature of 24°C and relative humidity of 50% were provided throughout the study. Evaporative cooling tests on the coated surfaces were compared to the plain surface. Tested dual-scale porous coatings enhanced evaporative heat transfer significantly, compared to that of the plain surface, due to the effective wicking of water to the entire heated area. With particle size increase, both the wickability and dryout heat flux were significantly increased. The dryout heat flux with the particle size of 114 µm was 3.2 times higher than that with the particle size of 27 µm. At the fixed particle size of 70 µm the dryout heat flux increased as thickness increased, which resulted in the maximum dryout heat flux of 10.6 kW/m² and the maximum heat transfer coefficient of 251 W/m²K at the coating thickness of 1200 µm.     

包覆吸水膜的管式蒸发散热器性能的实验研究

应用间接蒸发散热的原理,在空调冷凝器表面包覆吸水膜,利用水蒸发带走热量.这样蒸发面积达到了最大值,并且能够通过毛细力自动补充蒸发的水分.空调冷凝器中热工质的温度和热容比间接散热器中的一次空气大,能够提高蒸发表面温度,提高蒸发量,进而提高散热效率.通过对通有热水的表面覆盖吸水纸膜的单铜管的实验研究,得出了该方式的传热系数以及水膜的导热系数,证明了该散热方式较空调冷凝器空气强制对流和其它蒸发散热方式的优势.     

包覆吸水膜的管式蒸发散热器性能的实验研究

应用间接蒸发散热的原理,在空调冷凝器表面包覆吸水膜,利用水蒸发带走热量.这样蒸发面积达到了最大值,并且能够通过毛细力自动补充蒸发的水分.空调冷凝器中热工质的温度和热容比间接散热器中的一次空气大,能够提高蒸发表面温度,提高蒸发量,进而提高散热效率.通过对通有热水的表面覆盖吸水纸膜的单铜管的实验研究,得出了该方式的传热系数以及水膜的,导热系数,证明了该散热方式较空调冷凝器空气强制对流和其它蒸发散热方式的优势.     

Heat transfer characteristics of falling film evaporation on horizontal tube arrays

A falling film heat transfer test facility has been built for the measurement of falling film evaporation in a vacuum of about 1000 Pa. At this condition, only convective evaporation occurred in the liquid film. The Reynolds numbers of falling film over a range from 21.6 to 108.1 were tested on six-tube arrays made of enhanced or smooth tubes. Results show that the tubes with both enhanced outer and inner surfaces give high heat flux. Besides, as the Reynolds number increases, the heat transfer enhancement ratio of falling film evaporation decreases. A semi-analytical correlation is established to predict the heat transfer coefficients of falling film evaporation on smooth tube arrays, considering the contributions of partially dryout and fully wet regimes, respectively. For enhanced tubes, the heat transfer enhancement ratios to the smooth tubes were also correlated.     

Characterisation of the spray cooling heat transfer involved in a high pressure die casting process

A systematic experimental study was conducted to examine the heat transfer characteristics from the hot die surface to the water spray involved in high pressure die casting processes. Temperature and heat flux measurements were made locally in the spray field using a heater made from die material H-13 steel and with a surface diameter of 10 mm. The spray cooling curve was determined in the nucleate boiling, critical heat flux, as well as the transition boiling regimes. The hydrodynamic parameters of the spray such as droplet diameters, droplet velocities, and volumetric spray flux were also measured at the position in the spray field identical to that of the test piece. Droplet size and velocity distribution were measured using a PDA system. A new empirical correlation was developed to relate the spray cooling heat flux to the spray hydrodynamic parameters such as liquid volumetric flux, droplet size, and droplet velocity in all heat transfer regimes. The agreement between experimental data and predicted results is satisfactorily good.     

Experimental and theoretical study of axial dryout point for evaporation from V-shaped microgrooves

Experiments are carried out in a specially designed cell to study the onset and propagation of dryout point on a chemically machined microgrooved silicon surface with pentane as the coolant liquid. The axial temperature distribution is accurately measured as a function of the heat input and inclination of the substrate. The comparison between the dry (without liquid) and wet (with liquid) temperature profiles is used to locate the dryout point. The axial flow of an evaporating thin liquid film through a V-shaped microgroove with an appreciable inclination angle and varying evaporative heat flux is theoretically investigated. The nonlinear governing equations are solved numerically to predict the onset, location and propagation of the dryout point. The predictions from the theoretical analysis are successfully compared with the experimental results.     

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The lowest surface temperature possible for the existance of spray evaporative cooling is determined experimentally to be a linear function of the impinging spray mass flux. A conduction-controlled analytical model of droplet evaporation gives fairly good agreement with experimental measurements at atmospheric pressure. At reduced pressures droplet evaporation rates are decreased significantly such that an optimum operating pressure exists for each desired surface heat flux. The initiation of the ‘Leidenfrost state’ provides the upper surface temperature bound for spray evaporative cooling.     

Falling Films on Arrays of Horizontal Tubes with R-134a,Part I: Boiling Heat Transfer Results for Four Types of Tubes

A new falling film heat transfer test facility has been built for the measurement of local heat transfer coefficients on a vertical array of horizontal tubes, including flow visualization capabilities, for use with refrigerants. Presently, the facility has been used for evaporation tests on four types of tubes at three tube pitches and three nominal heat flux levels for R-134a at 5°C. A new method for determining local heat transfer coefficients using hot water heating has been applied, and test results for a wide range of liquid film Reynolds numbers have been measured for arrays made of plain, Turbo-BII HP, Gewa-B, and High-Flux tubes. The results show that there is a transition to partial dryout as the film Reynolds number is reduced, marked by a sharp falloff in heat transfer. Above this transition, the heat transfer coefficients are nearly insensitive to the film Reynolds number, apparently because vigorous nucleate boiling is always seen in the liquid film. The corresponding nucleate pool boiling data for the four types of tubes were also measured for direct comparison purposes. Overall, about 15,000 local heat transfer data points were obtained in this study as a function of heat flux, film Reynolds number, tube spacing, and type.     

Spray evaporative cooling to achieve ultra fast cooling in runout table

Spray evaporative cooling, in lieu of conventional laminar jet impingement cooling, has potential to achieve the anomalously high strip cooling rate of Ultra Fast Cooling – 300 °C/s for a 4 mm thick carbon steel strip – in Runout Table of Hot Strip Mill. In the present study, evaporation time of a single droplet impinging on a hot carbon steel strip surface has been analytically evaluated as a function of droplet diameter from fundamental heat transfer perspective based on the premise that a spray can be considered as a multi-droplet array of liquid at low spray flux density. Droplet evaporation time thus evaluated has been used to estimate strip cooling rate achievable in Runout Table of Hot Strip Mill by spray evaporative cooling. The proposed analytical model predicts that it is indeed possible to achieve the ultra-high cooling rate of Ultra Fast Cooling by spray evaporative cooling by suitable reduction of droplet size. A general analytical expression has also been developed to estimate critical droplet size to achieve Ultra Fast Cooling as a function of steel strip thickness. Predictions of the analytical model have been validated using CFD simulation with a modified Discrete Phase Model.     

Cooling enhancement in an air-cooled finned heat exchanger by thin water film evaporation

A theoretical analysis on the cooling enhancement by applying evaporative cooling to an air-cooled finned heat exchanger is presented in this work. A two-dimensional model on the heat and mass transfer in a finned channel is developed adopting a porous medium approach. Based on this model, the characteristics of the heat and mass transfer are investigated in a plate-fin heat exchanger with the interstitial surface fully covered by thin water film. Assuming that the Lewis number is unity and the water vapor saturation curve is linear, exact solutions to the energy and vapor concentration equations are obtained. The cooling effect with application of evaporative cooling was found to be improved considerably compared with that in the sensible cooler. This is because the thermal conductance between the fin and the air increases due to the latent heat transfer caused by the water evaporation from the fin surface. It is also found that the cooling enhancement depends greatly on the fin thickness. If the fin is not sufficiently thick, the cooling enhancement by the evaporative cooling decreases since the fin efficiency drops considerably due to the water evaporation from the fin surface. The fin thickness in the evaporative cooler should be increased larger than that in the sensible cooler to take full advantage of the cooling enhancement by the water evaporation.     

Falling Films on Arrays of Horizontal Tubes with R-134a,Part II: Flow Visualization,Onset of Dryout,and Heat Transfer Predictions

In tune with the falling film evaporation heat transfer test results described in Part 1, flow visualization of the boiling process and intertube flow mode transitions from droplet to column and sheet flows have been observed, and the onset of dryout results were obtained. A new empirical approach to describe falling film evaporation with a dominance of nucleate boiling that takes into account the onset of dryout has been proposed and is applicable to plain and enhanced tubes. The method predicts most of the current local measurements for R-134a to within ±20% for conditions without dryout (the desired design condition) and has also been extended to cover conditions with partial dryout. Furthermore, a criterion for predicting the onset of dryout as a function of heat flux has also been proposed for the four types of tubes.     

自由表面摩擦和蒸发对过冷下降液膜传热的影响

从理论上对下降液膜在自由表面上存在反向剪切力和蒸发散热情况下的换热特性进行了分析,得到了膜厚、换热系数的无量纲关系式,讨论了剪切力、液膜雷诺数、壁面热流、蒸发率对流动和传热的影响。     

A theoretical study of evaporative heat transfer in high‐velocity two‐phase flow of air–water in a small vertical tube

A theoretical study was performed to investigate the evaporative heat transfer of high‐velocity two‐phase flow of air–water in a small vertical tube under both heating conditions of constant wall temperature and constant heat flux. A simplified two‐phase flow boundary layer model was used to evaluate the evaporative heat transfer characteristics of the annular two‐phase flow. The analytical results show that the gravitational force, the gas–liquid surface tension force, and the inertial force are much smaller than the frictional force and hence can be neglected for a small tube. The evaporative heat transfer characteristics of the small tube with constant wall temperature are quite close to those of the small tube with constant heat flux. The mechanism of the heat transfer enhancement is the forced convective evaporation on the surface of the thin liquid film. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(5): 430–444, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10110     

Monodisperse Spray Cooling of Small Surface Areas at High Heat Flux

Spray cooling is an effective method to remove high heat fluxes from electronic components. To understand the physical mechanisms, this work studies heat transfer rates from single and dual nozzle distilled water sprays on a small heated surface (1.3 mm × 2 mm). Thermal ink jet atomizers generate small droplets, 33 μm diameter, at known frequencies, leading to controlled spray conditions with a monodisperse stream of droplets interacting with the hot surface. Of particular interest in this work is the dissipated heat flux and its relation to the liquid film thickness, the surface superheat, and the cooling mass flow rate. Experimental results show the heat flux scales to the cooling mass flow rate. In comparison to published spreading–splashing correlations, these experiments indicate that the drops impinge on the liquid film and spread without generating splashing, leading to high-efficiency stable heat transfer. Surface temperatures range from 120 to 140°C. In addition, the liquid film thickness is investigated in relation to the heater superheat and a stable thin film is seen at superheats beyond 20°C. The efficiency of the spray system is inversely related to the film thickness and may be due to ejection of liquid from the surface due to bursting of vapor bubbles.     

Evaporation heat transfer of liquid nitrogen on microstructured surface at high superheat level

Liquid nitrogen, as a coolant, is generally applied in cell vitrification cryopreservation. It takes heat from the carrier with cell samples through its violent evaporation on the carrier surface. As a result, the temperature of the carrier plunges dramatically. This article focuses on the unsteady evaporation heat transfer characteristics of liquid nitrogen on a microstructured surface etched into the frozen carrier surface at a high superheat level. The heat flux and evaporation heat transfer coefficient of liquid nitrogen were investigated using a lumped capacitance method. The experimental results showed that the cooling rate of the thin film evaporation on the microstructured surface is obviously higher than that of pool boiling, which is currently being used for cell cryopreservation. The heat flux and the evaporation heat transfer coefficient work together to present a parabolic trend with the superheat decreasing during this heat transfer process. Besides, the microstructure of the surface has an important effect on the evaporation heat transfer of liquid nitrogen. The larger the thin film evaporation zone is, the higher the heat transfer coefficient is. The current investigation results in a cell cryopreservation method through vitrification with relatively low concentrations of cryoprotectants.     

Effects of film evaporation on laminar mixed convection heat and mass transfer in a vertical channel

A numerical study of finite liquid film evaporation on laminar mixed convection heat and mass transfer in a vertical parallel plate channel is presented. The influences of the inlet liquid mass flow rate and the imposed wall heat flux on the film vaporization and the associated heat and mass transfer characteristics were examined for air-water and air-ethanol systems. Predicted results obtained by including transport in the liquid film are contrasted with those where liquid film transport is neglected, showing that the assumption of an extremely thin film made by Tsay and Yan (Wärme- und Stoffübertragung 26, 23–31 (1990)) is only valid for a system with a small liquid mass flow rate. Additionally, it is found that the heat transfer between the interface and gas stream is dominated by the transport of latent heat associated with film evaporation. The magnitude of the evaporative latent heat flux may be five times greater than that of sensible heat flux.     

Dryout analysis of overloaded microscale capillary-driven two-phase heat transfer devices

Dryout occurrence at high heat input is one of the detrimental factors that limit the thermal efficiency of a phase-change heat transfer device. In this work, we demonstrate that by employing visualization method, the dryout occurrence of an elongated liquid droplet in a transparent evacuated microscale two-phase flow device can be scrutinized. The circulation of liquid from the condenser to the evaporator is driven by the capillary action which is the primary limitation that governs the maximum heat transport capability of the device. When the evaporation rate exceeds the circulation rate of condensate, dryout will take place in the evaporator end. The propagation of dryout lengths can be accurately determined directly from visualization and a more accurate evaluation of the dryout length compared to the conventional method by measuring the axial temperatures has been developed. By quantifying the performance indicators of the cooling device over a wide range of operating conditions, including the underloaded and overloaded operations, the observation of dryout occurrence in this study correlates highly with the anticipated heat transfer characteristics of a phase-change heat transfer device. This study provides essential insights, particularly on the overloaded conditions, to the design of a microscale two-phase heat transfer device.     

Effect of copper surface wettability on the evaporation performance: Tests in a flat-plate heat pipe with visualization

The effects of copper surface wettability on the evaporation performance of a copper mesh wick were experimentally studied in an operating flat-plate heat pipe. Different degrees of wettability were obtained by varying the exposure times in air after the wicked plates were taken out of the sintering furnace. Three different working fluids: water, methanol and acetone, which possess different figures of merit, were investigated at the same volumetric liquid charge. The surface wettability was quantified by the static contact angle of sessile water drops on a flat copper surface. While the static contact angles of water drops varied from 10° to 40° for different degrees of wettability, the methanol and acetone drops still fully wetted the copper surface. A two-layer 100 + 200 mesh copper wick, 0.26 mm in thickness, was sintered on a 3 mm-thick copper base plate. A glass plate was adopted as the top wall of the heat pipe for visualization. Uniform heating was applied to the base plate near one end, and a cooling water jacket was connected at the other end. With increasing heat load, the evaporative resistance decreased with liquid film recession until a critical heat load showing the minimum evaporative resistance. Afterwards, partial dryout began from the front end of the evaporator. With decreasing wettability, the evaporating water film receded faster with increasing heat load and the critical heat loads were significantly reduced. In contrast, the critical heat loads for methanol and acetone seemed hardly affected by different wettability conditions. The minimum evaporative resistances, however, remained unaffected by surface wettability for all the three working fluids.     

Heat Transfer Enhancement of Spray Cooling on Flat Aluminum Tube Heat Exchanger

This study provides an experimental analysis on the heat transfer performance of a flat aluminum tube microchannel heat exchanger with spray cooling. The effects of water spraying rate, airflow rate, and relative humidity were investigated. The test results show that the heat transfer performance increased with increasing the water spraying rate but without the penalty of increased flow resistance at low spray conditions. This effect is further enhanced by increasing the water spraying rate. However, when the spraying rate is high, part of the nonevaporated drops attached to the fin surface and formed a liquid film, which caused the flow passage to become narrower. Further increase in the spraying rate resulted in part of the flow passages being blocked by the nonevaporated water drops and caused a region of poor heat transfer. The friction coefficient jumped drastically at this condition. This phenomenon deceased gradually with increasing airflow rate. High inlet air humidity resulted in the water accumulation phenomenon appearing at lower water spraying rates. The evaporative cooling effect decreased and flow friction increased. The test results just described show that the water spray is able to significantly improve the air-side heat transfer performance. The optimum spray rate for each airflow rate must be carefully determined.     

水平管降膜蒸发器传热传质实验研究

水平管降膜蒸发器因传质传热系数高而被广泛应用于淡化水处理中。搭建了水平管降膜蒸发传热实验台,通过对实验结果的归纳,得到了水平管降膜蒸发器的蒸发量随喷淋密度、蒸发温度、热通量的变化规律及热量利用率随蒸发温度的变化规律。结果表明,热通量范围不同时,蒸发量随喷淋密度的变化规律不同;蒸发量随热通量的增大而增大,随蒸发温度的增大而增大;热量利用率随蒸发温度的增大而增大。