Heat Flux Correlation for Spray Cooling in the Nonboiling Regime

An experimental facility was developed to investigate the nonboiling heat transfer performance of water spray cooling. The effects of mass flux and wall temperature on heat transfer coefficient and heat flux were experimentally studied. It was found that heat transfer coefficient increased with the increasing of mass flux and wall temperature. Generalized correlations were developed for the Nusselt number related to wall temperature and the average Nusselt number as a function of the spray Reynolds number and the nondimensional temperature with an absolute error of 4% and 15% when the Reynolds number is more than 440. Compared with the data of Oliphant et al., it was observed that the usage field of the correlations could be extended to Reynolds number greater than 240.     

Next Generation Spray Cooling: High Heat Flux Management in Compact Spaces

For many years, spray cooling has been known as a promising technology for the removal of high heat fluxes. However, that promise has yet to be fully realized. This work presents a current understanding of the mechanisms of spray cooling and its limitations. To address these limitations, a novel spray nozzle array is described that allows for scalable, high-performance cooling. However, fluid management remains a challenging concern that may limit the application of this technology.     

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.     

Numerical Solution of Periodic Heat Transfer in an Anisotropic Cylinder Subject to Asymmetric Heat Flux

Abstract

The problem of heat conduction in a two-dimensional anisotropic cylinder subject to asymmetric and periodic heat flux distribution on the outer wall is solved numerically. The dimensional analysis of the problem reveals that the heat conduction is a function of five nondimensional parameters: nondimensional frequency (α), cylinder outer to inner radius ratio (R₂), Biot number (Bi), orthotropicity factor (K₁), and anisotropicity factor (K_r1). A systematic study of the effect of each parameter is carried out over the influential range for each parameter. The results show that, depending on the combination of these parameters, the magnitude and/or phase of heat conduction in an anisotropic cylinder can be significantly different from those of an orthotropic and isotropic cylinder when subjected to the same externally imposed heat flux distribution.     

Field Computation for an Axial Flux Permanent-Magnet Synchronous Generator

An analytical method to determine the no-load and armature reaction magnetic fields of a single-sided, axial flux permanent-magnet synchronous generator without armature core is presented. Laplace's equation is solved in the rectangular coordinate system to give the scalar magnetic potentials, using a Fourier series method. For computation of the armature reaction field, a multi-current-sheet model is employed in order to account for the distributed nature of armature conductors in the axial direction. Finite element analysis and experimental results on a prototype machine are presented for verifying the accuracy of the proposed method.      

Heat Transfer Analysis of Functionally Graded Hollow Cylinders Subjected to an Axisymmetric Moving Boundary Heat Flux

The transient heat transfer analysis of functionally graded (FG) hollow cylinders subjected to a distributed heat flux with a moving front boundary on its inner surface is presented. The heat flux is assumed to be axisymmetric, and its front boundary moves along the axis of the cylinder. A method composed of the finite element and differential quadrature methods is employed to discretize the governing equations in the spatial domain. After demonstrating the convergence and accuracy of the method, the effects of different parameters on the temperature distribution and time history of the temperature at different points of FG cylinder are investigated.     

Evaluation of Jet Impingement,Spray and Microchannel Chip Cooling Options for High Heat Flux Removal

Thermal management for high heat flux removal from microelectronic chips is gaining critical importance in many earth-based and space-based systems. Heat fluxes greater than 1 MW/m² (100 W/cm²) have already been realized in high-end server applications, while cooling needs in next generation chips and advanced systems such as high-power electronics and electrical systems, pulsed power weapons systems, solid-state sensors, and phased-array radars are expected to reach 5–10 MW/m² (500–1000 W/cm²). After evaluating the contributions from different thermal resistances in the chip-to-ambient thermal path, this paper presents a critical review and research recommendations for three prominent contending technologies: jet impingement, spray cooling, and microchannel heat sinks.     

应用灰污热流计监测燃煤锅炉炉膛灰污结渣的动态过程

针对煤粉中混入造纸黑液后易结渣高碱燃料的结渣特性,将自制的水冷灰污热流计应用于工业性试验中,结合电子探针进行分析。提出灰渣沉积的3层沉积层———初始沉积层、一次沉积层和二次沉积层,并从宏观角度、微观结构、微区成分3方面给予论证。这有助于锅炉沾污结渣机理的研究。此外,通过数据采集仪,获取灰污热流计探头上温度信号,可以掌握热流密度随沾污结渣的动态变化,能够有效地监测炉内沾污结渣的发展,优化吹灰,在工程应用和试验分析中具有一定作用。图7表3参8     

无内热源下R134a喷雾冷却瞬态过程研究

搭建以R134a为制冷工质的闭式喷雾冷却试验台,试验研究喷雾冷却瞬态传热过程,建立了准确描述其传热过程的试验曲线,分析了蒸发压力对传热性能的影响,并阐述了每个瞬态传热阶段的传热机理。试验蒸发压力变化范围为0.207~0.331 MPa,流量范围为0.140~0.164 L/min。结果表明:膜态沸腾区在瞬态冷却过程中所占时间最长,且表面温度冷却速率保持在0.10 ℃/s,热流密度维持在20 W/cm2以下,故穿越膜态沸腾区的耗时决定着喷雾冷却瞬态过程的冷却速率;增加蒸发压力,可以提升冷却速率,当蒸发压力从0.207 MPa增加到0.331 MPa时,表面温度从130 ℃冷却至30 ℃所需的时间从508 s降至381 s;喷雾冷却瞬态过程在过渡沸腾区存在表面温度突变点,随着蒸发压力提高,突变点对应温度增加。     

高热流密度量热探针的设计与实验

以热等离子体射流冲击平板传热的热流密度测量为背景,分析比较了文献中报道过的各种测量高热流密度的量热探针的结构及实验方法;结合特定实验条件,设计了新型量热探针并对其测量高热流密度的方法误差进行了分析,描述了实际测量采用的方法,给出若干初步实验结果。     

Correlation to Predict the Maximum Heat Flux of a Vertical Closed-Loop Pulsating Heat Pipe

The objective of this study is to experimentally investigate the effect of various parameters on the maximum heat flux of a vertical closed-loop pulsating heat pipe (CLPHP) and the inside phenomena that cause maximum heat flux to occur. A correlation to predict the maximum heat flux using the obtained results was also established. Quantitative and qualitative experiments were conducted and analyzed. A copper CLPHP and a transparent high-temperature glass capillary tube CLPHP were used in the quantitative and qualitative experiments. From the study, it was found that when the internal diameter and number of meandering turns increased, the maximum heat flux increased. However, when the evaporator section length increased, the maximum heat flux decreased. The maximum heat flux of a CLPHP occurs due to the dry-out of liquid film at the evaporator section. This occurs after a two-phase working fluid circulation changes flow pattern from countercurrent slug flow to co-current annular flow, because the vapor velocity increases beyond a critical value. A correlation to predict the maximum heat flux obtained from this study was developed.     

高功率直流电弧等离子体流动传热过程的数值模拟

金刚石薄膜在现代工业和军事中具有重要应用价值,但是其制备过程中仍然存在着诸多问题,影响了其产业化发展。本文采用数值模拟的方法对影响金刚石薄膜制备的关键因素直流电弧等离子体的流动传热过程进行了研究,获得了等离子体流动与传热过程中的流场分布、温度分布、电离度分布等有关参量的分布状况,为掌握和控制电弧等离子体流动传热过程提供了理论依据和指导。     

Laminar Flow and Heat Transfer to a Non-Newtonian Fluid in an Entrance Region of a Square Duct with Prescribed Constant Axial Wall Heat Flux

Abstract

Forced-convection heat transfer information as a function of the pertinent nondimensional numbers is obtained numerically for laminar incompressible non-Newtonian fluid flow in the entrance region of a square duct with simultaneously developing temperature and velocity profiles for constant axial wall heat flux with uniform peripheral wall temperature. The power-law model characterizes the non-Newtonian behavior.

Finite-difference representations are developed for the equations of the mathematical model, and numerical solutions are obtained assuming uniform inlet velocity and temperature distributions. Results are presented for local and mean Nusselt numbers as functions of the Graetz number and the Prandtl number in the entrance region. Comparisons are made with previous analytical work for Newtonian fluids. The results show a strong effect of the Prandtl number on the Nusselt numbers with fully developed and uniform velocity profiles representing the lower and upper limits, respectively. The results provide a new insight into the true three-dimensional character of the pseudoplastlc fluid flow in the entrance region of a square duct and are accurate.     

Heat Transfer Characteristics of Gaseous Flows in Micro-Channel with Negative Heat Flux

Two-dimensional compressible momentum and energy equations are solved to obtain the heat transfer characteristics of gaseous flows in micro-channels with constant heat flux for which the value is negative for no-slip flow. The numerical methodology is based on the Arbitrary-Lagrangian-Eulerian method. The computations are performed for channels with constant heat flux ranging from ?10⁴ to ?10² W/m². The channel height ranges from 10 to 100 μ m and the aspect ratio of the channel height and length is 200. The stagnation pressure is chosen such that the exit Mach number ranges from 0.1 to 0.7. The outlet pressure is fixed at the atmosphere. The wall and bulk temperatures in micro-channels with negative heat flux are compared with those of positive heat flux cases obtained in our previous work and also those of the incompressible flow in a conventional sized channel. In the case of fast flow, temperatures normalized by heat flux have different trends whether heat flux value is positive or negative. A correlation for the prediction of the wall temperature of the gaseous flow in the micro-channel is proposed. The rarefaction effect is investigated for the cases of channel height of 10 μ m with slip boundary conditions. The magnitudes of viscous dissipation term and compressibility term are also investigated. The effect of each term on heat transfer characteristics is discussed.     

Dual-Phase-Lag Heat Conduction in Finite Slabs With Arbitrary Time-Dependent Boundary Heat Flux

Applying a constant or transient heat flux on a plane slab is a common technique in microelectronics technology and material processing, including laser patterning, micromachining, and laser surface treatment processes. Although Fourier's law is typically very precise for evaluating temperatures in solids, a number of experimental observations suggest the existence of non-Fourier transient conduction in these applications. Since the dual-phase-lag (DPL) model of heat conduction can be compatible with the hypothesis of local equilibrium thermodynamics (as shown here), the effects of temperature gradient relaxation time on the non-Fourier hyperbolic conduction in a finite slab subjected to an arbitrary time-dependent surface heat flux is examined by this model. The combination of diffusion- and wave-like features in heat conduction process is properly monitored by the DPL model for two types of heat flow regimes, namely, gradient precedence and flux precedence. The results indicate considerable deviations between the predictions of these regimes.     

Critical Heat Flux Tests for an Application of the Three-Pin Fuel Test Loop in HANARO

Critical heat flux (CHF) tests in a three-rod bundle were carried out, and a CHF database was established in order to obtain an operational license for the three-pin fuel test loop (FTL) in HANARO (High-flux Advanced Neutron Application Reactor). Two kinds of CHF tests are currently being performed for the CANDU and PWR-type fuel test loop. In this study, an experimental work on the CHF tests for the CANDU-type fuel test loop is explained, and a new CHF prediction methodology is proposed. In all, 108 experimental data points have been obtained, and the data are analyzed and compared with the available CHF correlations for a bundle or an annulus geometry. The 1986 AECL look-up table with a bundle correction factor and the Doerffer's correlation for annuli are compared with the present CHF data. It is found that the 1986 AECL look-up table with a bundle correction factor results in a better prediction than the Doerffer's correlation for annuli geometry. A three-pin correction factor is developed to account for the geometric effects of the three-rod bundle and to improve the prediction accuracy. It is concluded that the best estimate thermal hydraulic system code, MARS 3.0, which uses the same look-up table for a CHF prediction, can be used for a safety analysis of the CANDU-type three-pin FTL to obtain a license if it is corrected by the developed three-pin correction factor.     

Thermal Residual Stresses in One-Directional Functionally Graded Plates Subjected to In-Plane Heat Flux

This study carries out the transient thermal residual stress analyses of functionally graded clamped plates for different in-plane material compositions and in-plane heat fluxes. The heat conduction and Navier equations representing the two-dimensional thermoelastic problem were discretized using the finite-difference method, and the set of linear equations were solved using the pseudo singular value method. Both in-plane temperature distributions and the heat transfer period were affected considerably by the compositional gradient. The type of in-plane heat flux had a minor effect on the temperature profile, but on the heat transfer period. The high stress levels appeared in the ceramic-rich regions. The normal and equivalent stresses exhibited a sharp change in the plates with ceramic-rich as well as metal-rich compositions, and the concentrated on a narrow ceramic layer. A smooth stress variation was achieved through the graded region with a balanced composition of ceramic and metal-phases, and the stress discontinuities disappeared. The in-plane shear stress was negligible. The equivalent stress exhibited a linear temporal variation for both constant and sinusoidal heat fluxes, but a nonlinear variation for the exponential heat flux. In case the heat flux is applied along the metal edge (metal-to-ceramic plate) instead of the ceramic edge, the displacement and stress components exhibited similar distributions to those of a ceramic-to-metal plate but in the opposite direction. As a result, the distribution of in-plane material composition affects only normal stress distributions, whereas the peak stress levels occur in the ceramic-rich regions. Since the normal stresses concentrate along a narrow ceramic layer for ceramic-rich or metal-rich compositions, a balanced in-plane material composition distribution of ceramic and metal would be useful to avoid probable local ceramic fracture or damage.     

Quick Algebraic Estimate of the Thickness of Insulation for the Design of Process Pipelines with Allowable Heat Losses to Ambient Air

Adequate control of the total heat losses from in-tube fluid streams to neighboring ambient air is an important problem in heat transfer engineering. In this regard, the primary goal of this article is to demonstrate that an iterative solution of a nonlinear algebraic equation might allow thermal design engineers to estimate quickly the thickness of insulation of round tubes. The calculation methodology begins with the adoption of a powerful 1-D extended lumped energy model in favor of the customary 2-D differential energy model. The principal advantage of the former model is its ability to produce concise analytic expressions for the axial variation of the mean bulk temperature and also for the total heat transfer in the entire length of the tube. Particular attention was given to realistic situations in industry that account for laminar or turbulent velocities of single-phase viscous fluid flows in horizontal tubes rejecting heat by natural or forced cross flow of the ambient air. The total heat transfer to the air is the constraint design parameter in the set of design parameters. Once the magnitude of the total heat transfer was prespecified, the thickness of insulation of the round tube was easily computed numerically, exploiting the fixed-point iteration procedure for the solution of an adjoint nonlinear algebraic equation. Starting with a judicious guess of a root of the nonlinear algebraic equation, the correct root was surprisingly obtained in two or three iterations, thus furnishing immediately the required size of the thickness of the insulation annulus.     

Experimental Validation of a Thermal Model of Counterflow Microchannel Heat Exchangers Subjected to External Heat Flux

The effect of uniform external heat flux on the effectiveness of counterflow microchannel heat exchangers is experimentally studied in this article for validating an existing thermal model. The model validated in this study is a one-dimensional model previously developed by the same authors. The model is validated to be independent of microchannel profile, hydraulic diameter, and heat capacity ratio. For studying the effect of microchannel profile, experiments are conducted under balanced flow conditions using trapezoidal and triangular microchannels with approximately equal hydraulic diameter of 278.5 μm and 279.8 μm, respectively. The influence of hydraulic diameter on the thermal model is studied using a trapezoidal microchannel with hydraulic diameter of 231 μm and 278.5 μm. Experiments are conducted under unbalanced flow conditions, with a heat capacity ratio of 0.5, using the trapezoidal microchannel of hydraulic diameter of 278.5 μm. Deionized water is used as the fluid in all experiments. The hot and cold fluid effectiveness is studied and the theoretical predictions and experimental results are found to be in excellent agreement. Thus, the model validated in this article can be used for accurately modeling microchannel heat exchangers irrespective of the microchannel hydraulic diameter, profile, and heat capacity ratio.     

恒热流时污垢对管内对流换热过程热力学性能影响的分析

基于热力学第一、二定律,在恒热流工况下分析了污垢对管内对流换热过程热力学性能的影响,提出了一项在恒热流工况下反映污垢对管内流换热过程热力学性能影响的指标-无因次熵产相对增加数;讨论了管内流体雷诺数（无污垢时）和无因次热流密度等参数对无因次熵产相对增加数的影响。研究结果表明,该指标不仅能反映污垢对管内传热过程的影响,而且能反映污垢对管内流动过程的影响,而由污垢层导热所引起的熵产在管内传热过程总的熵产中占有重要的地位,同时,还把结果与恒壁温时的有关结果进行了比较。