The theory of advanced multi-layer thin film heat transfer gauges

This paper describes the theory of a transient method of measuring heat transfer rate to metal substrates coated with an electrical insulator, using thin film resistance thermometers. This builds on the already well-established system which uses semi-infinite insulating substrates. It is intended that the new technique will have application in rotating turbine test rigs, since there is at present a lack of suitable instrumentation which can be easily manufactured, and which does not interfere with the flow. The new system described here shows that multi-layer substrate gauges can be used. This paper presents analyses of layered gauges and gives sample predictions and calibrations.     

Effects of wall slip and temperature jump on heat and mass transfer characteristics of an evaporating thin film

A new mathematical model is developed to predict heat and mass transport characteristics of the evaporating thin film. The model considers effects of velocity slip and temperature jump at the solid-liquid interface, disjoining pressure, and surface tension. Three-dimensional nonequilibrium molecular dynamics simulations for coupling between the momentum and heat transfer at the nanoscale solid-liquid interface are performed to obtain the slip length and interfacial thermal resistance length. It is found that both slip length and interfacial thermal resistance length decrease significantly with the decreasing interface wettability of the liquid to the wall. Velocity slip and temperature jump at the solid-liquid interface intend to reduce the superheat degree of the evaporating thin film, and thus result in a sharp decrease of the heat and mass transport characteristics of the evaporating thin film. It is also noted that velocity slip and temperature jump at the solid-liquid interface show a more pronounced effect as the superheat degree increases.     

Heat transfer mechanisms in thin film with laser heat source

The present study deals with the effect of laser radiation on the propagation phenomenon of a thermal wave in a very thin film subjected to a symmetrical heating on both sides. Pulsating laser heating is modelled as an internal heat source with various time characteristics. The Cattaneo heat flux law together with the energy conservation equation is solved by a numerical technique based on explicit scheme, i.e., MacCormack’s predictor–corrector scheme. Results are obtained for the time history of heat transfer behaviour before and after symmetrical collision of wave fronts from two sides of a film. The study concludes (1) if the absorption coefficient of the continuously-operated- and pulsating-laser heat source increases, temperature overshoot causes in a very thin film within a very short period of time, and (2) the overshoot and oscillation of thermal wave depend on the frequency of the heat source time characteristics. This trend becomes minor in a thick film.     

Investigation of heat transfer and bubble dynamics in a boiling thin liquid film flowing over a rotating disk

Nucleate boiling heat transfer and bubble dynamics in a thin liquid film on a horizontal rotating disk were studied. A series of experiments were conducted to determine the heat transfer coefficient on the disk. At low rotation and flow rates, vigorous boiling increased the heat transfer coefficients above those without boiling. Higher rotational speeds and higher flow rates increased the heat transfer coefficient and suppressed boiling by decreasing the superheat in the liquid film. The flow field on the disk, which included supercritical (thin film) flow upstream of a hydraulic jump, and subcritical (thick film) flow downstream of a hydraulic jump, affected the type of bubble growth. Three types of bubble growth were identified. Vigorous boiling with large, stationary bubbles were observed in the subcritical flow. Supercritical flow produced small bubbles that remained attached to the disk and acted as local obstacles to the flow. At low rotational rates, the hydraulic jump that separated the supercritical and subcritical regions produced hemispherical bubbles that protruded out of the water film surface and detached from the disk, allowing them to slide radially outward. A model of the velocity and temperature of the microlayer of water underneath these sliding bubbles indicated that the microlayer thickness was approximately 1/25th of that of the surrounding water film. This microlayer is believed to greatly enhance the heat transfer rate underneath the sliding bubbles.     

Analysis of Evaporation Heat Transfer of Thin Liquid Film in a Capillary of Equilateral Triangular Cross—Section

IntroductionThe study on evaPoration heat transfer in caPillmpbodies is imPOrtant for design and development of higIilyefficient heat transfer equipment, such as heat pipe. P.C.W and Y.K. Kao[i] stUdied the interline heattransfer coefficients of an evaPorating wetting film.L.SoloVyev and S.A. KovalevI2l discussed the mechAnsmabout evaPoration of a liquld film from a porous sdse.A. Schonberg and P.C. Wayner['l developed Shae's(l953)['l evaPoration heat transfer model that based ontem…     

纳米流体颗粒沉积对薄液膜蒸发换热的影响

通过建立基于动量守恒和能量守恒的数学模型,比较纳米流体不同程度沉积造成表面粗糙度变化下,薄液膜的蒸发换热特性变化,分析得出表面粗糙度变化对薄液膜蒸发换热的影响。     

Effect of the plate thermal resistance on the heat transfer performance of a corrugated thin plate heat exchanger

Two‐dimensional conjugate conduction/convection numerical simulations were carried out for flow and thermal fields in a unit model of a counter‐flow‐type corrugated thin plate heat exchanger core. The effects of the thermal resistance of the solid plate, namely the variation of the plate thickness and the difference of the plate material, on the heat exchanger performance were examined in the Reynolds number range of 100<Re<400. Higher temperature effectiveness was obtained for a thicker plate at any Reynolds number, which was a unique feature of corrugated thin plate geometry. Detailed discussions on the thermal fields revealed that restricting the heat conduction along the plate by making the plate thinner or choosing a low thermal conductivity material causes a larger plate temperature variation along the plate, and, consequently, a smaller amount of thermal energy exchanged between two fluids. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(3): 209–223, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20110     

Experimental research of high‐temperature regenerative heat transfer

A small‐scale high‐temperature air combustion experimental system was set up. The effects of the switching period on the heat transfer characteristics of a regenerator were discussed, and an optimal switching period was suggested. The temperature distribution in the combustion chamber was analyzed and the emission of NO_X and CO with the operational parameters was summarized. The decomposability of CF₄ was primarily studied. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(5): 359–368, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20118     

Numerical study on fluid flow and heat transfer in the thin liquid film involving a hydraulic jump

The fluid flow and heat transfer in a thin liquid film are investigated numerically. The flow is assumed to be two-dimensional laminar, and surface tension effects at the exit are considered. The most important characteristic of this flow is the existence of a hydraulic jump through which the flow undergoes a very sharp and discontinuous change. In the present study, a simplified model of a free liquid jet impinging on a plane is considered. An arbitrary Lagrangian–Eulerian (ALE) method is used to describe the moving free boundary, and the fractional step method (FSM) based on the streamline upwind Petrov–Galerkin (SUPG) finite element method is used for the time-marching iterative solution. The numerical results obtained by solving the unsteady full Navier–Stokes equations are presented for plane and radial flows with constant wall temperature. © 1999 Scripta Technica, Heat Trans Asian Res, 28(1): 18–33, 1999     

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。     

Experimental validation of a numerical model for heat transfer in vacuum glazing

Flat vacuum glazings consisting of a narrow evacuated space between two glass panes separated by an array of small support pillars have been fabricated. A guarded hot box calorimeter was designed and constructed to measure their heat transfer coefficients. Experimental measurements of temperatures and rates of heat transfer were found to be in very good agreement with those predicted using a developed finite element model. A method for determining the heat transfer coefficient of the evacuated gap has been established and comparisons are made between the measured and predicted glass surface temperature profiles of the exposed glass area and the heat transfer coefficients of the total glazing system in order to validated the model.     

Evaporation heat transfer of steady thin film in micro capillary tubes of micron scale

This paper reports that the heat transfer mechanism of phase change in a capillary tube belongs to liquid film conduction and surface evaporation. The surface evaporation is influenced by vapor temperature, vapor‐liquid interfacial temperature, and vapor‐liquid pressure difference. In the vapor‐liquid flow mechanism, flow is effected by both the gradient of disjoining pressure, and the gradient of capillary pressure. The mechanism of vapor‐liquid interaction consists of the shear stress caused by momentum transfer owing to evaporation, and frictional shear stress due to the velocity difference between vapor and liquid. In the model presented for a capillary tube, the heat transfer, vapor‐liquid flow, and their interaction are more comprehensively considered. The thin film profile and heat transfer characteristics have close relations with a capillary radius and heat transfer power. The results of calculation indicate that the length of the evaporating interfacial region decreases to some extent with decreasing capillary radius and increasing heat transfer power. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 513–523, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ).DOI 10.1002/htj.10050     

Interface temperature and heat transfer in forced convection laminar film condensation of binary mixtures

Based on numerical solutions of the balance equations accurate empirical correlations for filmwise condensation in forced convection flow over a horizontal flat plate are presented. The correlations are designed to be also correct for the limiting case of zero and infinite condensation rate. They are applied to determine the interface temperature between liquid and vapour phase and hence the condensation rate and heat flux. The results are in excellent agreement with those from numerical solutions of the balance equations. It turns out that the usually adopted film theory for the vapour-phase mass transfer overestimates the size of a heat exchanger.     

Conduction based calibration of handmade platinum thin film heat transfer gauges for transient measurements

Heat transfer measurement using thin film gauges (TFG) is the most prevalently used technique for determination of surface heat flux. They are best suited for short duration transient surface temperature measurements and typically used in the applications where convection is a dominant mode of heat transfer such as gas turbine engines, high speed flights etc. However, in few interdisciplinary research areas, there are practical issues and difficulties in exposing the gauges for convection based measurements. These present investigations are aimed at exploring the possibility of using thin film gauges for short duration conduction based transient measurements with pure conduction mode of heat transfer. A simple calibration set-up has been used to supply known heat flux of different magnitudes to the thin film gauges that are fabricated in-house with platinum as sensing element and pyrex as an insulating substrate. Experimentally recorded temperature signals from the gauges are compared with simulated temperature histories obtained through finite element analysis. Convoluted integral of one-dimensional heat conduction equation is used to predict the surface heat flux and compared with input heat loads. The presently developed calibration setup is seen to be very useful for conduction based measurements of thin film gauges.     

Effect of salt additives on film boiling heat transfer and mechanism of quenching temperature rise

A high‐temperature stainless‐steel sphere was immersed into various salt solutions to investigate the film boiling behavior at vapor film collapse. The film boiling behavior around the sphere was observed with a digital video camera. Both surface temperature of the sphere and solid–liquid contact behavior were measured. Results of the experiment showed that salt additives enhanced condensation heat transfer, and the observed vapor film was thinner. Furthermore, the frequency of direct contact between the sphere surface and coolant increased. The quenching temperature increased with increased salt concentration, and was highly correlated with ion molar concentration, which represents the density of ions regardless of the type of salt. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20327     

Research on a new heat transfer model for liquid film convective evaporation in annular flow

Based on the phenomenon of turbulence restraint in liquid‐vapor interface, an analytical model is proposed for annular flow with a velocity distribution. The liquid‐vapor interface affecting district mixing length model was amended, and a new liquid film convective evaporation heat transfer model at the annular flow was developed. Compared with the experimental data, the results show that the new model is better than the model based on full tube flow velocity distribution. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 524–530, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10051     

双向开缝翅片管换热器传热与阻力特性试验研究

开缝翅片管换热器是在平直翅片管换热器基础上发展而来的强化传热管型,在空调、电站间接空冷系统等的传热设备中具有广阔的应用前景。为获得运行工况变化对双向开缝翅片管换热器传热与阻力特性的影响,进行了模化试验研究,获得了在进口水温相同的条件下,随着试件进口风温的升高,传热系数和空气侧流动阻力均减小的规律;在进口风温相同的情况下,随着进口水温的升高,对传热系数的影响不大,可以忽略不计,而空气侧流动阻力随之增大。