Study of transient heat transfer and synchronized flow visualizations during sub-cooled flow boiling in a small aspect ratio microchannel

The challenges that microchannel flow boiling technology faces are the lack of understanding of underlying mechanisms of heat transfer during various flow boiling regimes and a dearth of analytical models that can predict heat transfer. This paper aims to understand flow boiling heat transfer mechanisms by analyzing results obtained by synchronously captured high-speed flow visualizations with local, transient temperature data. Using Inverse Heat Conduction Problem (IHCP) solution methodology, the transient wetted surface heat flux and temperature as well as heat transfer coefficient are calculated. These are then correlated with the visual data. Experiments are performed on a single microchannel embedded with fast response temperature sensors located (630 µm) below the wetted surface. The height, width and length of the microchannel are 0.42 mm, 2.54 mm and 25.4 mm respectively. De-ionized, de-gassed water is used as the working fluid. Two heat fluxes are tested at each of the mass fluxes of 182 kg/(m²s) and 380 kg/(m²s). Because of vapor confinement, slug flow is observed for the tested conditions. The present study provides detailed insights into the effect of various events such as passage of vapor slug, 3-phase contact line, partial-dry-out and liquid slug on transient heat transfer coefficient. Transient heat transfer coefficient peaks when thin film evaporation mechanism is prevalent. The peak value is influenced by the distance of bubble incipience as well as downstream events obstructing the flow. Heat transfer coefficient during the passage of liquid slug and 3-phase contact line were relatively lower for the tested experimental conditions.     

Local heat transfer from a hot plate to a water jet

Jet impingement boiling is very efficient in cooling of hot surfaces as a part of the impinging liquid evaporates. Because of its importance to many cooling procedures, investigations on basic mechanisms of jet impingement boiling heat transfer are needed. Until now, most of the experimental studies, carried out under steady-state conditions, used a heat flux controlled system and were limited by the critical heat flux (CHF). The present study focuses on steady-state experiments along the entire boiling curve for hot plate temperatures of up to 700°C. A test section has been built up simulating a hot plate. It is divided into 8 independently heated modules of 10 mm length to enable local heat transfer measurements. By means of temperature controlled systems for each module local steady-state experiments in the whole range between single phase heat transfer and film boiling are possible. By solving the two dimensional inverse heat conduction problem, the local heat flux and the corresponding wall temperature on the surface of each module can be computed. The measurements show important differences between boiling curves measured at the stagnation line and those obtained in the parallel flow region. At the stagnation line, the transition boiling regime is characterised by very high heat fluxes, extended to large wall superheats. Inversely, boiling curves in the parallel flow region are very near to classical ones obtained for forced convection boiling. The analysis of temperature fluctuations measured at a depth of 0.8 mm from the boiling surface enables some conclusions on the boiling mechanism in the different boiling regimes.     

An experimental study of the heat transfer in PS foam insulation

Heat transfer mechanisms in 14 samples of vacuum insulation panels (VIPs) are examined to reveal the influence of porous foam structure on VIP performance. The samples were produced by in-house equipment that was able to vary the foam structure by modulating the process temperature and pressure. Two parameters are proposed to describe the foam structure, namely, the broken cell ratio and the average cell size. Under a specific solid volume fraction, the average cell size shows a linear dependence on the broken cell ratio. Furthermore, the radiation and conduction heat transport data correlate well with these parameters. Radiation heat transfer increases as the broken cell ratio (cell size) increases, but solid conduction decreases as the broken cell ratio (cell size) increases. Consequently, an optimum broken cell ratio (cell size) exists such that the total heat transport is minimum under a specific solid volume fraction. However, the majority of VIP heat transfer is solid conduction. Solid conduction accounts for more than 80% of the total heat transport and is largely affected by the solid volume fraction. A rule of thumb for improving VIP performance is to reduce the solid volume fraction as much as possible to eliminate solid conduction, and maintain the cell size at an optimum value that is dependent on the solid volume fraction.     

瞬态耦合热弹塑性接触有限元分析方法

在弹性接触问题有限元混合法的基础上,把材料非线性和表面非线性两种迭代过程耦合,在瞬态温度场分析中将伽辽金法和向后差分法结合,并用混合法进行热接触迭代,把瞬态温度场分析和弹塑性接触分析耦合。提出了一种瞬态耦合热弹塑性接触有限元分析方法,并已成功地用于核容器的密封分析。     

Influence of surface topography in the boiling mechanisms

The present paper addresses the qualitative and quantitative analysis of the pool boiling heat transfer over micro-structured surfaces. The surfaces are made from silicon chips, in the context of pool boiling heat transfer enhancement of immersion liquid cooling schemes for electronic components. The first part of the analysis deals with the effect of the liquid properties. Then the effect of surface micro-structuring is discussed, covering different configurations, from cavities to pillars being the latter used to infer on the potential profit of a fin-like configuration. The use of rough surfaces to enhance pool boiling mainly stands on the arguments that the surface roughness will increase the liquid–solid contact area, thus enhancing the convection heat transfer coefficient and will promote the generation of nucleation sites. However, one should not disregard bubble dynamics. Indeed, the results show a strong effect of bubble dynamics and particularly of the interaction mechanisms in the overall cooling performance of the pair liquid–surface. The inaccurate control of these mechanisms leads to the formation of large bubbles and strong vertical and horizontal coalescence effects promote the very fast formation of a vapor blanket, which causes a steep decrease of the heat transfer coefficient. This effect can be strong enough to prevail over the benefit of increasing the contact area by roughening the surface. For the micro-patterns used in the present work, the results evidence that one can reasonably determine guiding pattern characteristics to evaluate the intensity of the interaction mechanisms and take out the most of the patterning to enhance pool boiling heat transfer, when using micro-cavities. Instead, it is far more difficult to control the appearance of active nucleation sites and the optimization of the patterns allowing a reasonable control of the interaction mechanisms and in particular of horizontal coalescence, when dealing with the patterns based on micro-pillars. Hence, providing an increase of the liquid contact area by an effective increase of the roughness ratio is not enough to assure a good performance of the micro-structured surface. Despite it was not possible to clearly evidence a pin–fin effect or of an additional cooling effect due to liquid circulation between the pillars, the results show a significant increase of the heat transfer coefficient of about 10 times for water and 8 times for the dielectric fluid, in comparison to the smooth surface, when the micro-patterning based on pillars is used.     

Heat transfer during transient compression: Measurements and simulations

Experiments have been performed to assess the utility of unsteady one-dimensional heat conduction modelling for the calculation of heat losses during a free piston compression process. Heat transfer measurements have been obtained within a gun tunnel barrel using surface junction thermocouple instrumentation. The gun tunnel was operated with a relatively heavy piston such that the shock waves induced by the piston motion were weak. Peak heat transfer values are estimated reasonably well by the unsteady one-dimensional model. However, overall quantitative agreement between the measurements and calculations has not been achieved at this stage, principally because the development of turbulent heat transport was not properly modelled. Received 21 September 2001 / Accepted 11 March 2002 – Published online 11 June 2002     

Thermal and mechanical analysis of material response to non-steady ramp and steady shock wave loading

Ramp wave experiments on the Sandia Z accelerator provide a new approach to study the rapid compression response of materials at pressures, temperatures and stress or strain rates not attainable in conventional shock experiments. Due to its shockless nature, the ramp wave experiment is often termed as an isentropic (or quasi-isentropic) compression experiment (ICE). However, in reality there is always some entropy produced when materials are subjected to large amplitude compression even under shockless loading. The entropy production mechanisms that cause deformation to deviate from the isentropic process can be attributed to mechanical and thermal dissipations. The former is due to inelasticity associated with various deformation mechanisms and the rate effect that is inherent in all the deformation processes and the latter is due to irreversible heat conduction. The main purpose of the current study is to gain insights into the effects of ramp and shock loading on the entropy production and thermomechanical responses of materials. Another purpose is to investigate the role of heat conduction in the material response to both the non-steady ramp wave and steady shock.Numerical simulations are used to address the aforementioned research objectives. The thermomechanical response associated with a steady shock wave is investigated first by solving a set of nonlinear ordinary differential equations. Using the steady wave solutions as the reference, the material responses under non-steady ramp waves are then studied with numerical wave propagation simulation. It is demonstrated that the material response to ramp and shock loading is essentially a manifestation of the interaction between the time scale associated with the loading and the intrinsic time scales associated with mechanical deformation and heat transfer. At lower loading rates as encountered in ramp loading, the loading path is closer to an isentrope and results in lower entropy production. The reasonable ramp rate to obtain a quasi-isentropic state depends on the intrinsic time scales of the dissipation mechanisms which are strongly material dependent. Thus shockless loading does not necessarily produce an isentropic response. Between two equilibrium states, heat conduction was shown to have significant effect on the temperature history but it contributes little to the overall temperature change if the specific heat remains constant. It also affects the history of entropy, but only the irreversible part of heat conduction contributes to the net entropy change. The various types of thermomechanical responses of materials would manifest themselves more significantly in terms of the thermal history than the mechanical history. Thus temperature measurement appears to be an important experimental tool in distinguishing the various mechanisms for the thermomechancial responses of the materials.     

Heat and mass transfer in direct contact hygroscopic condensation

A theoretical analysis of direct contact hygroscopic-condensation of cold vapor on hot films is presented. The condensation of the relatively low temperature, low pressure, vapors on a hot film of an hygroscopic brine solution may occur due to the reduced vapor pressure of a sufficiently concentrated solution. The driving force for condensation is the difference between the partial pressure of water in the brine and the partial pressure of the condensing water vapor. The condensation is also governed by simultaneous mass transfer mechanisms, due to a non-isothermal absorption, with a possible opposing thermal driving force in the condensing vapor phase. The overall performance is determined by the accumulating effects of the various resistances to heat and mass transfer. The present study is aimed to elucidate the controlling mechanisms associated with this absorption-condensation process, and suggest overall transfer rates at the laminar and turbulent flow regimes.     

Particle convective heat transfer near the wall in a supercritical water fluidized bed by single particle model coupled with CFD-DEM

Supercritical water fluidized bed (SCWFB) is a promising reactor to gasify biomass or coal. Its optimization design is closely related to wall-to-bed heat transfer, where particle convective heat transfer plays an important role. This paper evaluates the particle convective heat transfer coefficient (h_pc) at the wall in SCWFB using the single particle model. The critical parameters in the single particle model which is difficult to get experimentally are obtained by the computational fluid dynamics-discrete element method (CFD-DEM). The contact statistics related to particle-to-wall heat transfer, such as contact number and contact distance, are also presented. The results show that particle residence time (τ), as the key parameter to evaluate h_pc, is found to decrease with rising velocity, while increase with larger thermal boundary layer thickness. τ follows a gamma function initially adopted in the gas–solid fluidized bed, making it possible to evaluate h_pc in SCWFB by a simplified single particle model. The theoretical predicted h_pc tends to increase with rising thermal gradient thickness at a lower velocity (1.5 U_mf), while first decreases and then increases at higher velocity (1.75 and 2 U_mf). h_pc occupies 30%–57% of the overall wall-to-bed heat transfer coefficient for a particle diameter of 0.25 mm. The results are helpful to predict the overall wall-to-bed heat transfer coefficient in SCWFB combined with a reasonable fluid convective heat transfer model from a theoretical perspective.     

A heat transfer model for biological wastewater treatment system

A heat transfer model for predicting the water temperature of aeration tank in a biological wastewater treatment plant is presented. The heat transfer mechanisms involved in the development of the heat transfer model include heat gains from solar radiation and biochemical reaction and heat losses from evaporation, aeration, wind blowing and conduction through tank walls. Several empirical correlations were adopted and appropriate assumptions made to facilitate the model development. Experiments were conducted in the biological wastewater treatment plant of a chemical fiber company over a year's period. The operational, weather and temperature data were registered. The daily water temperature data were averaged over a month period and compared with the theoretical prediction. Excellent agreement has been obtained between the predicted and measured temperatures, verifying the proposed heat transfer model.     

Infrared visualization of thermal motion inside a sessile drop deposited onto a heated surface

Drop evaporation is a basic phenomenon but the mechanisms of evaporation are still not entirely clear. A common agreement of the scientific community based on experimental and numerical work is that most of the evaporation occurs at the triple line. However, the rate of evaporation is still predicted empirically due to the lack of knowledge of the governing parameters on the heat transfer mechanisms which develop inside the drop under evaporation. The evaporation of a sessile drop on a heated substrate is a complicated problem due to the coupling by conduction with the heating substrate, the convection/conduction inside the drop and the convection/diffusion in the vapor phase. The coupling of heat transfer in the three phases induces complicated cases to solve even for numerical simulations. We present recent experimental results obtained using an infrared camera coupled with a microscopic lens giving a spatial resolution of 10 μm to observe the evaporation of sessile drops in infrared wavelengths. Three different fluids fully characterized, in the infrared wavelengths of the camera, were investigated: ethanol, methanol and FC-72. These liquids were chosen for their property of semi-transparency in infrared, notably in the range of the camera from 3 to 5 μm. Thus, it is possible to observe the thermal motion inside the drop. This visualization method allows us to underline the general existence of three steps during the evaporating process: first a warm-up phase, second (principal period) evaporation with thermal-convective instabilities, and finally evaporation without thermal patterns. The kind of instabilities observed can be different depending on the fluid. Finally, we focus on the evolution of these instabilities and the link with the temperature difference between the heating substrate and the room temperature.     

Third heat‐transfer crisis with stepwise heat supply

The paper reports experimental results on heattransfer crises with stepwise heat supply to a heater, in which the metastable liquid decomposes in the form of vaporization fronts. Data on the dynamics of heattransfer crises under saturation and underheating conditions are given. It is shown that below the vapor formed during propagation of vaporization fronts, a liquid microlayer is absent.     

Parametric study of thermal constriction resistance

Thermal contact resistance is due to imperfect contact of two bodies at the interface. It plays an important role in the dissipation of heat from electronic devices. The concept of individual heat flux tubes consisting of a single contact area and the corresponding gap which extends far in either solid was used in this study. The three dimensional conduction equation in the contact region was solved numerically for different shapes of gap and contact area and various thermal boundary conditions at locations far from the contact area. Constriction resistance defined as the ratio of the temperature difference across the contact surface to the rate of heat transfer through a heat flux channel was calculated for each case. The results have indicated that constriction resistance is strongly affected by the gap geometry, shape of contact area and certain end surface boundary conditions. The geometry dependence becomes more significant as the ratio of contact to total area becomes smaller. Given the fact that the shape of the contact region is highly unpredictable, the heat flux tube approach can hardly provide a reasonable estimate of the thermal contact resistance, unless the geometry of the contact region is properly modeled. Received on 5 January 1999     

Comparative investigation of operational performance characteristics of axially grooved and arterial heat pipes

The thermal performance characteristics of an axially grooved heat pipe (AGHP) and an arterial heat pipe (ArHP) sharing a similar external configuration are investigated. A mathematical model is developed to predict the capillary heat transfer limit for both heat pipes. The meniscus attachment point, contact angle and liquid–vapor interfacial shear stress are taken into account in this model. In particular, for predicting the ArHP dry-out, a novel model is proposed by introducing two different failure mechanisms. The results of the mathematical model are experimentally verified.     

Comparison of Pore-Level and Volume-Averaged Computations in Highly Conductive Spherical-Void-Phase Porous Materials

A study has been carried out to compare results obtained from pore-level simulations conducted on three-dimensional idealized spherical-void-phase geometric models to similar results obtained from a solver based on volume-averaging and local thermal non-equilibrium. The purpose of the comparison is to establish closure coefficients for the viscous and form drag terms in the volume-averaged momentum equations and the interstitial convective exchange coefficient required to couple the volume-averaged energy equations for the solid and fluid constituents. A method is also described for determining the solid-phase conduction shape factor, which is shown to be important for accurate volume-average simulation of highly conductive porous materials. The shape factor has been addressed in previous literature (using various terminology) and accounts in a bulk manner for resistance due to the elongated conduction path and for changes in the effective heat flow area along the conduction path. The conduction shape factor is a function of the geometry only and is found herein from a detailed comparison between pore-level and volume-averaged simulations of conjugate heat transfer. The conduction shape factor vastly improves volume-averaged predictions of the overall heat transfer and the temperature distributions in the porous material.     

Thermographic validation of a novel,laminate body,analytical heat conduction model

The two-region fin model captures the heat spreading behaviour in multilayered composite bodies (i.e., laminates), heated only over a small part of their domains (finite heat source), where there is an inner layer that has a substantial capacity for heat conduction parallel to the heat exchange surface (convection cooling). This resulting heat conduction behaviour improves the overall heat transfer process when compared to heat conduction in homogeneous bodies. Long-term heat storage using supercooling salt hydrate phase change materials, stovetop cookware, and electronics cooling applications could all benefit from this kind of heat-spreading in laminates. Experiments using laminate films reclaimed from post-consumer Tetra Brik cartons were conducted with thin rectangular and circular heaters to confirm the laminate body, steady-state, heat conduction behaviour predicted by the two-region fin model. Medium to high accuracy experimental validation of the two-region fin model was achieved in Cartesian and cylindrical coordinates for forced external convection and natural convection, the latter for Cartesian only. These were conducted using constant heat flux finite heat source temperature profiles that were measured by infrared thermography. This validation is also deemed valid for constant temperature heat sources.     

非傅立叶导热的最新研究进展

对迄今为止有关非傅立叶导热的研究成果进行了全面的综述,其中包括作者在该领域的最新研究进展:空心球体介质双曲线非傅立叶导热模型的分析求解,室温条件下多孔材料内非傅立叶导热的实验结果及数值模拟,非傅立叶导热的“瞬时薄层”模型,非傅立叶导热和非费克质量传递的耦合分析,非傅立叶导热的分子动力学模拟等．文中还对下一步的研究工作进行了展望.      

Natural convection in a porous medium coupled across an impermeable vertical wall with film condensation

This paper presents a theoretical study of thermofluid interaction between natural convection in fluid-saturated porous medium and film condensation, coupled through an impermeable vertical wall. The two heat transfer modes are analyzed separately. The solutions are matched on the wall. The complexion of this two-fluid problem is governed by a dimensionless interaction parameter which relates the heat transfer effectiveness of the two heat transfer mechanisms. The effect of this parameter on the flow and heat transfer is documented. Results regarding the overall heat transfer coefficient are obtained for a wide range of the independent parameters. Received on 19 January 1998     

Heat transfer to an unconfined ceiling from an impinging buoyant diffusion flame

Impinging flames are used in fire safety research, industrial heating and melting, and aerospace applications. Multiple modes of heat transfer, such as natural convection, forced convection and thermal radiation, etc. are commonly important in those processes. However, the detailed heat transfer mechanisms are not well understood. In this paper, a model is developed to calculate the thermal response of an unconfined nonburning ceiling from an impinging buoyant diffusion flame. This model uses an algorithm for conduction into the ceiling material. It takes account of heat transfer due to radiation from the fire source to the ceiling surface, and due to reradiation from the ceiling surface to other items. Using experimental data, the convective heat transfer coefficient at lower surface is deduced from this model. In addition, the predicted heat fluxes are compared with the existing experimental data, and the comparison results validate the presented model. It is indicated that this model can be used to predict radial-dependent surface temperature histories under a variety of different realistic levels of fire energy generation rates and fire-to-ceiling separation distance.     

Solidification and melting heat transfer to an unfixed phase change material (PCM) encapsulated in a horizontal concentric annulus

The solidification and melting process of an unfixed PCM between two isothermal concentric horizontal cylinders was investigated by experimental techniques and by a combined analytical and numerical method. During the solidification process concentric solid PCM layers form at both tube walls, growing slowly into the annulus. Assuming quasi-steady heat conduction, this process is described by a simple analysis. The melting PCM reveals a different behaviour. Due to gravitational forces the solid phase moves downwards. Experiments prove that the solid retains contact with the lower part of the outer tube as well as with the upper part of the inner tube. In this process thin liquid films form between the solid body and the heated walls and heat transfer by conduction is the dominating mechanism during melting. Heat transfer by natural convection causes the melting at the upper interface. There, the melting rates, however, are comparatively small. The theoretical approach and the numerical analysis are based on a balance of the pressure forces in the thin liquid films and of the gravitational force acting on the solid material. As a result melting rates and heat fluxes may be predicted. For practical application a Nusselt correlation is derived.