The critical heat transfer characteristics of an insulated or compound sphere considering heat radiation

Critical heat transfer from an insulated or compound sphere happen in ambient air with natural convection. Thus, heat radiation in such a situation must be considered because natural convection has only a small effect. It is found that the critical heat transfer characteristics considering heat radiation are very different from those neglecting heat radiation. The conventional critical heat transfer characteristics neglecting heat radiation only depend on sphere size, external convection heat transfer coefficient, and insulation conductivity. In addition to the above parameters, the critical heat transfer characteristics considering heat radiation are also related to the surface emissivities of the sphere and the insulation, the surrounding temperature, and the internal convection heat transfer coefficient.     

Effect of variable viscosity on free convection over a non-isothermal axisymmetric body in a porous medium with internal heat generation

Summary. A similarity solution is proposed for the analysis of the steady free convection boundary layers over a non-isothermal axisymmetric body embedded in a fluid saturated porous medium. The effect of temperature dependent viscosity on heat transfer rates in the presence of internal heat generation is investigated. The linearized version of the Arrhenius law for the temperature dependent viscosity is considered. It is shown that the heat transferred is more for a less viscous fluid.     

Convective heat transfer at a stretching sheet

Summary Analysis of convection flow and heat transfer of a viscous heat-generating fluid near an infinite vertical stretching surface is carried out. The effects of free convection and the presence of heat generation/absorption on the flow and heat transfer characteristics are considered. The equations of conservation of momentum, mass, and energy, which govern the flow and heat transfer problem, are solved numerically by using a variable order, variable step size finite-difference method. The numerical results obtained for the flow and heat transfer characteristics reveal many interesting behaviors. These behaviors warrant further study of the effects of free convection on the flow and heat transfer characteristics.     

Numerical simulation of forced flow in boundary layer and of heat transfer by non-Newtonian fluid in view of the temperature dependence of viscosity

Differences are determined in the degree of the effect made by the temperature dependence of viscosity of fluid on heat transfer and friction. The effect of the temperature factor depends on the direction of heat flux and on the index of nonlinearity of the medium; in so doing, the rheology makes no effect on relative heat transfer and friction. The temperature dependence of viscosity of fluid is self-similar relative to the Prandtl number and makes no effect on friction. Criterional equations are obtained for the calculation of local and average values of Nusselt number and coefficients of friction under conditions of laminar forced convection in the vicinity of a cooled plate.     

圆环腔内纳米流体自然对流的数值研究

采用Fluent软件对圆环封闭腔内的Ag-水纳米流体自然对流传热进行数值模拟,着重分析在不同瑞利数下Ag纳米颗粒的添加量和圆环内外壁半径比对圆环传热性能的影响.研究结果表明,随着瑞利数的增加,圆环间的换热强度不断加剧,换热由热传导逐渐向对流转变.添加纳米颗粒降低了换热性能,且随着颗粒浓度的增加换热效果不断恶化;同时,圆环半径比对换热有很大的影响,对一定的瑞利数而言随着半径比的减小,换热性能逐渐增强,且增大的趋势越来越显著.     

Theoretical study of hydrodynamics and heat transfer by free convection of non-Newtonian fluids near a cooled surface with regard for variable viscosity

We have made a theoretical study of the hydrodynamics and heat transfer of non-Newtonian fluids near a cooled isothermal surface by laminar free convection with regard for the change in the fluid viscosity with temperature. A power rheological model has been used. The solutions to the systems of differential equations for the boundary layer have been obtained numerically. It has been shown that the strongest influence in the considered kinds of convection is produced by the relative viscosity. Moreover, of great importance is the nonlinearity index of the medium. With increasing rheological parameter the influence of variable viscosity decreases. Criteria equations for calculating the local and mean Nusselt numbers and friction coefficients have been obtained. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 6, pp. 49–56, November–December, 2006.     

Natural convective heat transfer of suspensions of titanium dioxide nanoparticles (nanofluids)

This paper reports an experimental study on the natural convective heat transfer of nanofluids, an area in which little work has been carried out in the past. Aqueous-based titanium-dioxide nanofluids of various concentrations are formulated by using the two-step method and a high shear homogenizer is used to break large aggregates. Instead of the use of dispersant and/or surfactant, the electrostatic repulsion mechanism is adopted to stabilize nanoparticles. The resulting nanofluids are found to be very stable, although the actual measured particle size is much larger than the primary nanoparticle size. The stable nanofluids are then used for both the transient and steady-state heat transfer experiments under natural convection conditions. The results show that the presence of nanoparticles systematically decreases the natural convective heat transfer coefficient under the conditions of this study, which is an observation that contrasts with the previous expectation. Discussion of the results suggests that changes in the nanofluids' thermal conductivity and viscosity could not explain the observed decrease in the heat transfer coefficient, and particle-surface interactions may play an important role.     

Conjugate heat transfer by boundary-domain integral method

A novel approach to the numerical simulation of conjugate heat transfer is presented. Heat conduction in a solid is implicitly coupled with heat convection in viscous fluid flow. The frame of the solution is the Navier–Stokes equations set for viscous Newtonian fluid. A formulation for planar geometry is described in detail. The main advantage of the presented approach is implicit handling of the heat transfer conditions at the solid–fluid interface. Computed test examples include conjugate forced convection in a channel and conjugate natural convection in a cavity.     

Effect of convection heat transfer on the design of vapor-cooled current leads

A general optimization method for vapor-cooled current leads is presented with taking into account the effect of convection heat transfer and extended surfaces. This analytical work is considered as a unified design method, since one formulation calculates the minimum heat load and the corresponding optimal design condition for arbitrary heat transfer condition, spanning two limiting cases—the zero convection (or conduction-cooled leads) and the perfect heat transfer. It is clearly shown that the augmentation of the convective cooling can reduce the heat load to a certain extent, but the optimum lead parameter required to minimize the heat load for the finite heat transfer may not exist between the two limiting values. A new dimensionless parameter called the Ch number is introduced to conveniently incorporate the convection effect into the optimization. The present method is demonstrated for two specific lead designs that have been recently developed for 10 kA level of applications.     

封闭腔内MWCNT纳米流体自然对流传热的数值模拟

采用Fluent软件对封闭腔内纳米流体层流自然对流换热进行了数值模拟研究.重点分析了Ra数和纳米颗粒的体积分数对自然对流换热特性的影响.数值模拟结果表明:在机油中添加多壁碳纳米管(MWCNT)粒子并没有提高基液的自然对流传热特性;对于给定的Ra数下,随着纳米颗粒体积分数的增大,纳米流体的传热特性也随之减弱;对于给定的体积分数,随着Ra数的增大,纳米流体的传热特性显著增强,但纳米流体的传热性能比机油的要弱,且在同一体积分数下随着Ra数的增大,传热性能减弱的程度要减小.     

Constructal multi-scale structure for maximal heat transfer density

Summary.  This paper presents a new concept for generating the multi-scale structure of a finite-size flow system that has maximum heat transfer density–maximum heat transfer rate installed in a fixed volume. Laminar forced convection and parallel isothermal blades fill the volume. The spacings between adjacent blades of progressively smaller scales are optimized based on constructal theory: the goal is maximum heat transfer density. The smaller blades are installed in the fresh-fluid regions that sandwich the tips of the boundary layers of longer blades. The overall pressure difference is constrained. As the number of length scales increases, the flow rate decreases and the volume averaged heat transfer density increases. There exists a smallest (cutoff) length scale below which heat transfer surfaces are no longer lined by distinct (slender) boundary layers. Multi-scale flow structures for maximum heat transfer rate density can be developed in an analogous fashion for natural convection. The constructal multi-scale algorithms are deduced from principles, unlike in fractal geometry where algorithms are assumed. Received January 3, 2003 Published online: June 12, 2003     

Experimental study of laminar natural convection heat transfer from a vertical cylinder to slush nitrogen under constant heat flux conditions

Natural convection heat transfer from a vertical cylinder immersed in slush and subcooled liquid nitrogen and subjected to constant heat fluxes was investigated in order to determine the relative merits of slush nitrogen (SlN₂) for immersion cooling. A glass dewar was used as a test vessel in which a cylindrical heater was mounted vertically, and heat transfer measurements were carried out for SlN₂ and subcooled liquid nitrogen (LN₂) in the laminar flow range. The results revealed advantages of SlN₂ over subcooled LN₂ in natural convection cooling. The local temperatures of the heated surface surrounded by solid nitrogen particles are measured to increase at much slower rates than in subcooled LN₂, which is due to the latent heat of fusion of solid nitrogen. Even after the solid nitrogen particles surrounding the heater are apparently depleted, the average heat transfer coefficients for SlN₂ are still found to be greater than those for LN₂ with the improvement in heat transfer being larger for lower Grashof number regime. Our analysis also indicates that solid nitrogen particles in close proximity to heated surface do not discourage local convection due to the porous nature of SlN₂, making the heat transfer in SlN₂ more effective than in the case of solid–liquid phase change of nitrogen involving melting and conduction processes.     

Combined heat transfer under conditions of free convection

On the basis of the boundary-layer equations solutions are obtained for the problems of convective and combined heat transfer under conditions of free convection in transparent and gray media. The solution for convective heat transfer in a transparent medium is obtained by means of a Taylor expansion of the temperature function. Expressions for the radiative and convective components of combined heat transfer are also presented, and it is shown that these fluxes are interrelated quantities.     

Short-time heat impulse intensification of heat transfer in liquid nitrogen

The cryogenic technology deals with fluids produced from gases after liquefaction. Boiling of cryogenic fluids is frequently characterized by a hysteresis of their boiling curve. The present experimental work demonstrates an opportunity to intensify heat transfer in those fluids by means of short-term heat impulsion from a heater. The intensification takes place due to the impulse-induced transition of heat transfer regime from natural convection towards nucleate boiling. The process takes place when the impulse magnitude overcomes certain minimum value that was quantified experimentally. We also propose a theoretical expression for the minimum energy that is in agreement with the experimental data.     

Free convection and heat transfer in a near-supercritical fluid in a horizontal square cavity with lateral heating

The problem of natural convection and heat transfer of water is solved numerically under a pressure of 23 MPa near the critical isochore, in a square cavity with lateral heating. It is shown that natural convection under near-supercritical state of the liquid is implemented in microchannels and under conditions of microgravity. The data on heat transfer are generalized.     

Effect of counterflow mixed convection on the heat transfer in vertical liquid-metal heat exchangers

The results are shown here of an experimental and analytical study concerning the heat transfer between sodium and sodium-potassium alloy in counterflow heat exchangers with counterflow mixed convection in one of the ducts. It is shown that, at low Pe numbers, the heat transfer is determined by the effect of longitudinal heat overruns in the liquid.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 20, No. 6, pp. 995–1002, June,1971.     

国内外吸收式热泵强化传热传质研究综述

综述国内外对吸收式热泵强化传热传质研究的现状。目前主要的研究方向为新型强化管的开发、新型表面活性剂及强化吸收机制的研究,主要研究目的是如何增大传热面积与加强界面马拉格尼对流,以此提高传热传质系数。     

Investigation of turbulent heat transfer and nanofluid flow in a double pipe heat exchanger

In present study, heat transfer and turbulent flow of water/alumina nanofluid in a parallel as well as counter flow double pipe heat exchanger have been investigated. The governing equations have been solved using an in-house FORTRAN code, based on finite volume method. Single-phase and standard k-ε models have been used for nanofluid and turbulent modeling, respectively. The internal fluid has been considered as hot fluid (nanofluid) and the external fluid, cold fluid (base fluid). The effects of nanoparticles volume fraction, flow direction and Reynolds number on base fluid, nanofluid and wall temperatures, thermal efficiency, Nusselt number and convection heat transfer coefficient have been studied. The results indicated that increasing the nanoparticles volume fraction or Reynolds number causes enhancement of Nusselt number and convection heat transfer coefficient. Maximum rate of average Nusselt number and thermal efficiency enhancement are 32.7% and 30%, respectively. Also, by nanoparticles volume fraction increment, the outlet temperature of fluid and wall temperature increase. Study the minimum temperature in the solid wall of heat exchangers, it can be observed that the minimum temperature in counter flow has significantly reduced, compared to parallel flow. However, by increasing Reynolds number, the slope of thermal efficiency enhancement of heat exchanger gradually tends to a constant amount. This behavior is more obvious in parallel flow heat exchangers. Therefore, using of counter flow heat exchangers is recommended in higher Reynolds numbers.     

A numerical study on heat transfer of the nanofluid flow inside helical tube through the two-phase method

In this study, a new numerical investigation was carried out to study the heat transfer characteristics of nanofluid flow inside a copper helical tube under constant heat flux. A nanofluid with different particle weight concentrations of 0.5%, 1.0%, and 2.0% was used. The effects of different parameters such as Reynolds number, nanofluid particle concentration, and constant heat fluxes (1500 and 3800?W/m²) on heat transfer coefficient were studied. For validation, Nusselt number and convection heat transfer coefficient obtained from the numerical model was compared with the experimental results. Also, to verify the accuracy of the method, grid independency was studied for each heat flux. The observations showed that the heat transfer coefficient increased by using nanofluid instead of base fluid. In addition, the convection heat transfer coefficient performance improved by increasing the nanoparticles’ concentration. The results from the numerical simulation compared with the experimental data showed that this new numerical method has high accuracy and could correctly predict the heat transfer behavior of nanofluids with different weight particle concentrations under constant heat flux.     

Effect of discrete flow distribution on heat transfer at a vertical surface in natural convection

The influence of a discrete distribution of injection and suction of air on heat transfer at a vertical flat surface under free convection has been investigated experimentally. The results are presented together with empirical formulas for the heat transfer coefficients.