Heat transfer in a second order viscoelastic boundary layer flow at a stagnation point

The steady and transient heat transfer characteristics of a second order viscoelastic boundary layer flow at a stagnation point have been studied in this paper. The implicit cubic spline numerical procedure is used to solve the governing boundary layer equations. The details of the temperature profiles and wall heat flux rates have been graphically illustrated. The range of values of the Prandtl number was from 5 to 1000 while the Weissenberg number was varied from 0.1 to 0.3.     

Heat exchange in granular-bed tubes

The results of investigations of the wall and total coefficients of heat exchange of a granular bed with the tube wall is analyzed. The generalized dependences for calculation of these parameters, which have been checked in a wide range of experimental conditions, are obtained. The limiting values of the coefficient of wall heat exchange at small Reynolds numbers are established from the experiments of N. V. Antonishin and associates on nonstationary heat exchange in an unblown granular bed. The influence of the longitudinal thermal conductivity of the bed on the calculated values of the wall heat-exchange coefficient is shown.     

Unsteady heat transfer characteristics of a two dimensional laminar wall jet

An analysis is presented to investigate the transient heat transfer characteristics of a laminar plane wall jet. The assumption of incompressible fluid with constant viscosity coefficient has been made. The transient surface heat flux and details of the temperature field are obtained and have been presented graphically. An engineering estimate of the time required to reach steady state has been given. The range of Prandtl numbers investigated is from 0.01 to 1000.     

A review of free,forced and mixed convection in a saturated porous annulus

A computational study of heat transfer in a liquid-saturated porous annulus with a heated inner wall and a cold outer wall is reported here. Results are presented for heat transfer rates from the inner cylinder for a wide range of parameters which characterize free, forced and mixed convective regimes of flow. Both horizontal and vertical annulii are included in the study. Heat transfer results have also been obtained from a non-Darcy model for flow and these are compared to the Darcy solutions. Non-Darcy effects are taken to arise from inertial and viscous effects in the fluid phase and the variation of porosity near the solid wall. The approach of heat transfer results of an annulus with a large radius ratio towards those of an isolated circular cylinder is discussed. Limited data on transient convection is also presented in this work.     

Experimental Study of the Relation Between Heat Transfer and Flow Behavior in a Single Microtube

The flow boiling heat transfer in microchannels have become important issue because it is extremely high-performance heat exchanger for electronic devices. For a detailed study on flow boiling heat transfer in a microtube, we have used a transparent heated microtube, which is coated with a thin gold film on its inner wall. The gold film is used as a resistance thermometer to directly evaluate the inner wall temperature averaged over the entire temperature measurement length. At the same time, the transparency of the film enables the observation of fluid behavior. Flow boiling experiments have been carried out using the microtube under the following conditions; mass velocity of 105 kg/m² s, tube diameter of 1 mm, heat flux in the range of 10 ~ 380 kW/m² s, and the test fluid used is ionized water. Under low heat flux conditions, the fluctuations in the inner wall temperature and mass velocity are closely related; the frequency of these fluctuations is the same. However, the fluctuations in the inner wall temperature and heat transfer coefficient are found to be independent of the fluctuation in the mass velocity under high heat flux conditions.     

Effect of finite thermal conductivity of the separating wall on the performance of counterflow heat exchangers

Axial conduction is a major source of inefficiency in a compact counterflow heat exchanger. Any attempt to reduce axial conduction by using material of low thermal conductivity for the separating wall results in increased resistance to lateral heat flow, thereby reducing the overall thermal efficiency of the heat exchanger. The governing equations including axial conduction and lateral resistance due to the separating wall have been solved and an expression, for the overall efficiency of the heat exchanger has been derived in terms of relevant nondimensional parameters. Computed results have been presented which give the optimum thermal conductivity of the wall material.     

An experimental study of heat transfer characteristics of a two-phase nitrogen thermosyphon over a large dynamic range operation

Heat transport characteristics of a cryogenic two-phase nitrogen thermosyphon have been experimentally investigated in this study. The thermal resistance and the maximum heat transfer rate were mainly investigated over a wide dynamic range from near the triple point to the critical point. The experimental data suggests that the nominal thermal resistance does not have pressure dependence in the high pressure and high temperature region. The present experimental result is well explained by the theoretical prediction. From the experimental result of the operating limit of the thermosyphon, it is found that the maximum heat transfer rate is governed by the interaction between the vapor flow and the returning liquid film flow along the wall in the evaporator section, even near the critical point.     

重力铯热管等温性能研究

为了验证铯热管研制的关键技术,通过测量热管外壁温度,在定温条件下研究了重力铯热管的等温特性和启动性能;同时,分析了冷凝段长度对铯热管等温性能的影响。实验结果表明:当加热炉温度在330~630℃,铯热管均能正常启动;加热炉温度越高,启动越快;在该温区,铯热管具有优良的传热性能;然而,当冷凝段长度为300mm时,铯热管壁面温度出现锯齿状周期波动。从过热度的角度,分析了铯热管内部的沸腾相变传热机理;选择合适长度的冷凝段可避免周期性间歇沸腾的产生。实验结果同时也了证明铯热管在330~630℃温区可作为高效的传热元件,非常适合复现ITS-90国际温标锌凝固点。     

单侧受限管排自然对流的数值模拟

给出了具有 5,10,15,20 根管的管排在 103相似文献   

CFD study of forced convective heat transfer enhancement in a 90° bend duct of square cross section using nanofluid

In this paper, the forced convective heat transfer enhancement with nanofluids in a 90° pipe bend has been presented. Numerical investigation is carried out for the turbulent flow through the pipe employing finite volume method. The governing differential equations are discretized using hexahedral cells, and the resulting algebraic equations are solved using Commercial solver Fluent 6.3. In order to close the time averaged Navier–Stokes equations, the two-equation k–? turbulence model with a standard wall function have been used. The duct Reynolds number is varied in the range of 2,500–6,000. It is observed that the heat transfer is enhanced significantly by varying the volume fraction of the nanofluid. It is also found that the heat transfer is increased with Reynolds number. A strong secondary flow is observed due to the presence of the wall. Turbulent kinetic energy near outer wall is found to be higher than the inner wall of the bend. A comparative assessment for the heat transfer enhancement with different types of nanofluids is also presented. The computed results of area weighted average Nusselt numbers are validated with some of the existing literature.     

Effect of magnetic fields on heat convection inside a concentric annulus filled with Al2O3–water nanofluid

In the current study, forced convective heat transfer of an MHD fully developed laminar nanofluid between two concentric horizontal cylinders is investigated in the presence of a radial magnetic field. In contrast to a conventional no-slip condition at the surfaces, the Navier’s slip condition is considered at the surface to represent the non-equilibrium region near the surfaces. Employing the modified Buongiorno model, the conservative partial differential equations have been collapsed to two-point ordinary boundary value differential equations before being numerically solved. To consider the effects of thermal boundary condition on nanoparticle migration, two distinctive cases including constant heat flux at the outer wall and adiabatic inner wall (Case A) and constant heat flux at the inner wall with adiabatic outer wall (Case B) have been considered. Our results indicate that due to thermophoresis force, the distribution of nanoparticles was denser at the adiabatic wall for the case A which affects the local and the universal fluid flow and heat transfer characteristics. Moreover, inducing a radial magnetic field on the system, heat transfer rate was increased for the case A which had a decreasing effect on the case B. Finally, slip velocity at the walls enhances heat transfer rate for both cases.     

Behavior of Hydroxyl Groups in Quartz Glass during Heat Treatment in the Range 750–950°C

The concentration of hydroxyl groups in different types of quartz glass tubes, the OH concentration profile across the tube wall, and the kinetics of OH removal during heat treatment in the temperature range 750–950°C have been studied by Fourier transform IR spectroscopy. For TK-I tubes, produced by vacuum electromelting of quartz, followed by annealing in a hydrogen atmosphere, we have calculated the diffusion coefficient of OH groups in this temperature range and the activation energy for diffusion: 215 ± 10 kJ/mol. We have found heat treatment conditions that ensure a considerable decrease in the concentration of OH groups in quartz tubes that are used in the fabrication of reactors for the preparation of extrapure chalcogenide glasses.     

Heat Exchange in a Tube Filled with Granular Bed

Results of modeling of the process of heat transfer from a circular tube filled with granular bed with boundary conditions of the first, second, and third kind have been presented. The physical characteristics of the wall zone and the relative value of its thermal resistance have been determined based on an analysis of experimental data on the nonstationary heat exchange of the unblown granular bed. Recommendations on calculation of the heat exchange at elevated temperatures have been given.     

Pool Boiling with Non-condensable Gas in Microgravity: Results of a Sounding Rocket Experiment

Pool boiling experiments in microgravity have been performed in the Sounding Rocket Maser 11. A heated plate of 1cm ² was located at the bottom of a small cylindrical tank partly filled with a refrigerant Novec HFE7000 pressurized with Nitrogen. Experiments were performed at different reservoir pressures and wall heat fluxes. The wall heat flux and wall temperature were simultaneously measured during the experiment and the behavior of the bubbles on the heater was filmed with a video camera through the transparent wall of the reservoir. The presence of Nitrogen dissolved inside the liquid led to a strong Marangoni convection around the bubble. The effect of Marangoni convection and evaporation on the wall heat transfer is analyzed in function of the relative values of the wall temperature and saturation temperature.     

Unsteady attachment line flow on a flat plate with attached cylinder

The unsteady laminar incompressible boundary-layer attachment-line flow on a flat plate with attached cylinder with heat and mass transfer has been studied when the free stream velocity, mass transfer and surface wall temperature vary arbitrarily with time. The governing partial differential equations with three independent variables have been solved numerically using an implicit finite-difference scheme. The heat transfer was found to be strongly dependent on the Prandtl number, variation of wall temperature with time and dissipation parameter (for large times). However, the free stream velocity distribution and mass transfer affect both the heat transfer and skin friction.     

Natural convection on a thin vertical cylinder moving in a high-porosity ambient medium

An analysis has been performed to study the natural convection flow over a thin vertical cylinder which is moving with a constant velocity in a non-Darcy high-porosity ambient medium. Both constant wall temperature and constant heat flux conditions have been considered. The coupled non-linear parabolic partial differential equations have been solved numerically by using an implicit finite-difference scheme. The heat transfer is found to be significantly affected by the inertia and porosity parameters, and the Prandtl number, whereas the skin friction is weakly affected. The heat transfer for the constant heat flux case is more than that of the constant wall temperature case and this difference increases with the Prandtl number. The heat transfer increases with the buoyancy force, but the skin friction is slightly reduced.     

Finite difference analysis for developing laminar flow in circular tubes applied to forced and combined convection

The complete two-dimensional partial differential equations for developing laminar flow in a circular tube have been treated by a finite difference analysis. Property variation with temperature, especially that of viscosity, is allowed for in a flexible manner. The continuity and momentum equations, and then the energy equations, are solved by direct elimination at each axial step, and marching procedure used in the axial direction. A new technique is that the stepwise energy balance is rigidly satisfied throughout by using it as a constituent equation in place of the ‘explicit’ wall thermal boundary condition normally used. The analysis predicts the complete developing hydrodynamic and thermal fields, together with friction factors and heat transfer coefficients. It has been tested for a range of fluid velocity and thermal boundary conditions and for various fluids, including high viscosity oils, water and air. Data for constant wall heat flux have already been published. ^1,2 Predictions for constant wall temperature presented here are for forced and combined convection and are compared with experimental data of Test³ and Zeldin and Schmidt⁴.     

Initiation of convection flows in the wall granular layer in the problem of boiling of subcooled coolant

A computational model of boiling of a subcooled liquid in the wall granular layer has been developed on the basis of new experimental data on initiation and evolution of convective flows during nonstationary heating of the wall. The influence of the thermophysical properties of the batch elements on the temperature distribution in the wall region has been studied. The characteristic features of initiation of microconvection in a model cell have been revealed, and the impact of microconvection on the conditions of vapor bubble nucleation depending on the initial subcooling and the extent of the delivered heat flux has been established. New experimental data on limiting heat fluxes that cause microconvection have been obtained with the help of a gradient heat flux sensor for different combinations of the properties of the liquid and the particles of the granular layer model.     

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.     

Measuring local thermal fluxes during thermal exchange between a surface and a vortex structure

The effect of a vortex structure in a “dumped” flame on the intensity of heat exchange between this flame and a flat surface has been studied. An optical polarization technique is proposed for the visualization and measurement of an inhomogeneous temperature field at the surface and inside the heat exchanger wall. It is shown that the gas circulation in large vortex cells leads to an increase in the local thermal fluxes. In the range of Reynolds numbers 50400, a thermal flux from the “dumped” flame to a flat heat exchanger surface is 20–30% greater than that for an oncoming laminar flame.