Film boiling on a vertical plate in subcooled helium II

Film boiling heat transfer coefficients were measured on 10, 30 and 50 mm long vertical plates in subcooled He II for bulk liquid temperatures from 1.8 to 2.1 K. A film boiling model on a vertical plate in subcooled He II was presented based on convection heat transport in the vapor film, radiation heat transport, and heat transport in He II. The numerical solutions of the model were obtained and an equation which can express the numerical solutions within ±5% difference was derived. The equation predicted well the experimental data for lower ΔT range but significantly under-predicted the data for higher ΔT. A correlation of film boiling heat transfer including radiation contribution was presented by modifying the equation based the experimental data. This correlation can describe the experimental data within ±20% difference.     

Performance of extended surface from a cryocooler for subcooling liquid nitrogen by natural convection

Natural convection of subcooled liquid nitrogen under a horizontal flat plate is measured by experiment. This study is motivated mainly by our recent development of cryocooling systems for HTS power devices without any forced circulation of liquid nitrogen. Since the cold surface of a GM cryocooler is very limited, the cooling plate immersed in subcooled liquid nitrogen is thermally anchored to the cryocooler located at the top in order to serve as an extended surface. A vertical plate generating uniform heat flux is placed at a given distance under the cooling plate so that subcooled liquid may generate cellular flow by natural convection. The temperature distributions on the plates and liquid are measured during the cool-down and in steady state, from which the heat transfer coefficients are calculated and compared with the existing correlations for a horizontal surface with uniform temperature. A fair agreement is observed between two data sets, when the heat flux is small or the plate temperatures are relatively uniform in horizontal direction. Some discrepancy at higher heat flux is explained by the cellular flow pattern and the fin efficiency of the extended surface, resulting in the non-uniformity of the horizontal plate.     

Natural convection from a semi-infinite flat plate inclined at a small angle to the horizontal in a saturated porous medium

Summary The free convection boundary layer on a semi-infinite heated flat plate which is inclined at a small angle to the horizontal in a saturated porous medium is considered. When the plate is inclined upwards, series solutions, one valid near the leading edge and the other valid at large distances from it, are obtained. Sufficient terms in the leading edge solution are taken so that the two asymptotic solutions may be joined. When the plate is inclined downwards the series solution valid near the leading edge is again obtained and sufficient terms are again employed to investigate the nature of the boundary layer separation. There is no evidence of a singularity at the separation point and a mathematical explanation of the behaviour at separation is presented.With 10 Figures     

Numerical analysis of two-dimensional steady-state and transient heat transfer in a parallel duct filled with pressurized He II

Analysis on steady-state and transient heat transfer on a flat plate at the middle of a parallel duct immersed in He II was performed for bath temperatures from 1.8 to 2.1 K at 101.3 kPa. Two-dimensional computer code named SUPER-2D developed by the authors based on the two-fluid model and the theory of mutual friction was used. Steady-state critical heat flux (CHF) and the time lag from the application of a step heat input to λ transition, that is called a lifetime, were obtained numerically for various step heat fluxes and for the channel gaps from 2 to 20 mm. Effect of the gap restriction on the CHF and the lifetime were clarified. The solutions were compared with the experimental data for the ducts with the same structures and the corresponding conditions. They agreed well with the experimental data. The heat transport mechanism in the parallel duct was clarified.     

A study of unsteady-state operating conditions of a supersonic inlet by the RANS/ILES method

Using the hybrid RANS/ILES method, a flow was investigated under unsteady-state conditions in a rectangular mixed-compression supersonic inlet at M_d = 2. The computations were made for Mach numbers of the incident flow M₀ = 1.8, 2, and 3. The geometry of two variants was investigated, viz., with the boundary layer bleed system and without it. Calculations were performed on the meshs containing (1.69–1.78) × 10⁶ cells. The flow rate through the supersonic inlet was varied within a wide range. Under most of the conditions investigated, the flow in the duct was unsteady as a consequence of separation of the boundary layer upon its interaction with the shock waves. For all geometric variants investigated and M₀, the throttle characteristics were constructed by the averaged flow parameters as well as the dependences of the flow rate through the boundary layer bleeding system and the static pressure pulsation level on the air inlet throttle ratio. Comparison of the computed results with the experimental data showed a good agreement in terms of both the averaged flow parameters and the pulsation characteristics.     

套管结构对翅片管气化器结霜特性的影响

结霜会对不同类型气化器造成长效性能的衰减.为研究套管结构对强化传热与表面结霜效果的影响,本文搭建了实验台,在普通传热管内加装套管,以液氮为工质,根据不同套管内径(Φ6、Φ8、Φ10 mm)及入口流量,设计了12组实验工况进行研究,获得翅片管表面不同测点处温度、霜层厚度及翅片管出口流体温度等参数.结果表明:入口流量为2....     

Heat and mass transfer from a laminar humid air stream to a plate at subfreezing temperature

Processes involving the transfer of heat from a humid air stream in laminar flow to a horizontal plate with the simultaneous deposition of frost is of importance in a variety of refrigeration equipment. The accumulating frost layer impedes the heat flow to the cooling surface. The present study is carried out to determine both theoretically and experimentally the factors that influence, frost formation on a cold surface and to correlate the Nusselt and Sherwood numbers with these factors.Experiments were conducted on a 600 mm long, flat horizontal plate, the Reynolds number and the relative humidity of the impinging air stream were varied from 30 000 to 140 000 and from 40% to 60% respectively, while the average surface temperature was near - 18°C. Empirical correlations of the Nusselt number and Sherwood number are presented.     

Some peculiarities of the asymmetric Graetz problem

A numerical simulation and an analysis of the steady state forced convection heat transfer with plane laminar flow confined by two parallel plates that are kept at constant but different temperatures are presented. We name this heat transfer configuration shortly the asymmetric Graetz problem. The essential features of the asymmetric in comparison to the symmetric Graetz problem are the reversal of the heat flux and the jump of the Nusselt number from positive to negative region at the plate having the temperature closer to the fluid inlet temperature. These phenomena occur at different axial positions, which depend on the thermal asymmetry and the fluid inlet conditions. The numerical results agree excellently with an analytical solution obtained in terms of Kummer confluent hypergeometric function and Hermite polynomials.     

Heat and mass transfer from a laminar humid air stream to a plate at subfreezing temperatureTransfert de chaleur et de masse entre un écoulement laminaire d'air humide et une plaque à une température inférieure au point de congélation

Processes involving the transfer of heat from a humid air stream in laminar flow to a horizontal plate with the simultaneous deposition of frost is of importance in a variety of refrigeration equipment. The accumulating frost layer impedes the heat flow to the cooling surface. The present study is carried out to determine both theoretically and experimentally the factors that influence, frost formation on a cold surface and to correlate the Nusselt and Sherwood numbers with these factors.Experiments were conducted on a 600 mm long, flat horizontal plate, the Reynolds number and the relative humidity of the impinging air stream were varied from 30 000 to 140 000 and from 40% to 60% respectively, while the average surface temperature was near - 18°C. Empirical correlations of the Nusselt number and Sherwood number are presented.     

The effect of oscillating flow on a horizontal dilute-phase pneumatic conveying

ABSTRACT

A horizontal dilute-phase pneumatic conveying system using vertically oscillating soft fins at the inlet of the gas–particle mixture was studied to reduce the power consumption and conveying velocity in the conveying process. The effect of different fin lengths on horizontal pneumatic conveying was studied in terms of the pressure drop, conveying velocity, power consumption, particle velocity, and intensity of particle fluctuation velocity for the case of a low solid mass flow rate. The conveying pipeline consisted of a horizontal smooth acrylic tube with an inner diameter of 80 mm and a length of approximately 5 m. Two types of polyethylene particles with diameters of 2.3 and 3.3 mm were used as conveying materials. The superficial air velocity was varied from 10 to 17 m/s, and the solid mass flow rates were 0.25 and 0.20 kg/s. Compared with conventional pneumatic conveying, the pressure drop, MPD (minimum pressure drop), critical velocities, and power consumption can be reduced by using soft fins in a lower air velocity range, and the efficiency of fins becomes more evident when increasing the length of fins or touching particles stream by the long fins. The maximum reduction rates of the MPD velocity and power consumption when using soft fins are approximately 15% and 26%, respectively. The magnitude of the vertical particle velocity for different lengths of fins is clearly lower than that of the vertical particle velocity for a non-fin conveying system near the bottom of the pipeline, indicating that the particles are easily suspended. The intensities of particle fluctuation velocity of using fins are larger than that of non-fin. The high particle fluctuation energy implies that particles are easily suspended and are easily conveyed and accelerated.     

Numerical analysis for two-dimensional transient heat transfer from a flat plate at one-side of a duct containing pressurized He II

A numerical analysis of two-dimensional transient heat transfer produced by step heat inputs to a flat plate located at one end of a rectangular duct containing pressurized He II was performed. The computer code, SUPER-2D, that is the two-dimensional heat transfer code for He II developed by Tatsumoto et al. [1] based on the two fluid model and the theory of the mutual friction was used for the calculation. The time lag from the application of the step heat input to the occurrence of the λ transition, which is called a lifetime, was obtained for various values of step heat flux for bulk liquid temperatures ranging from 1.8 to 2.0 K. The calculations were made for three rectangular ducts having different cross-sectional area. The ratios of the cross-sectional area of the duct to the heated surface area, A_d/A_h, were 1.0, 1.6 and 2.0. Effect of the A_d/A_h on the lifetime was clarified. The solutions agreed with the experimental data by Shiotsu et al. [2] for the ducts with the same structures and the corresponding conditions. It was confirmed that the computer code, SUPER-2D, can describe transient heat transport in He II as well as steady-state one [1].     

中厚板加速冷却和直接淬火时冷却能力研究

为了确定加速冷却或直接淬火时实现预期的冷却速率所需的对流换热系数,利用MSC.MARC有限元分析软件对Q345B中厚钢板冷却过程中温度场进行了数值模拟计算.确定了实现直接淬火条件下不同厚度(20 mm)钢板的理论极限冷却速率所需的对流换热系数为15 000 W/(m2·℃),并分析了冷却速率与对流换热系数、钢板厚度之间关系.研究表明,对于同一厚度、材质中厚钢板,其冷却速率随对流换热系数的增加而增大.超快速冷却或直接淬火时,带钢冷却速率随对流换热系数增加而显著增加;对流换热系数大于15 000 W/(m2·℃)时,厚度(30 mm)钢板的冷却速率基本不变,达到其物理极限冷却速率;换热系数增加,厚度方向上温度梯度增加.     

Linear stability of natural convection in a multilayer system of fluid and porous layers with internal heat sources

The onset of thermal natural convection in a horizontal multilayer system consisting of a homogeneous porous layer sandwiched between two fluid layers has been simulated by an accurate numerical method. The porous and fluid layers include uniform heat sources. Flow in the porous medium has been governed by Darcy–Brinkman’s law. On the other hand, the Navier–Stokes equations with the Boussinesq approximation have ruled over the clear fluid layers. The lower and upper rigid surfaces are assumed to be fixed at the equal temperatures T _L and T _U. The eigenvalues and eigenfunctions of the linear stability analysis have been solved by utilizing the compound matrix method (CMM). The CMM reaches accurate results in a very efficient manner. Moreover, the method removes the stiffness from the equations of the stability system. The results indicate that the onset of convection and the nature of convection cells depend on the relative depths of layers. It has been observed that the thickness of the lower fluid layer increases the critical Rayleigh number of the upper fluid layer and stabilizes it.     

Prediction and Control of Internal Condensing Flows in the Experimental Context of their Inlet Condition Sensitivities

The reported experimental results involve fully condensing flows of pure FC-72 vapor on a horizontal condensing surface (316 stainless steel) which is the bottom surface (wall) of a rectangular cross-section duct of 2 mm height, 15 mm width, and 1 m length. The sides and top of the duct are made of clear plastic. The experimental system in which this condenser is used is able to control steady-in-the-mean (termed quasi-steady) mass flow rate, exit pressure, and wall cooling conditions. It has been found that, with the condenser mean (time averaged) inlet mass flow rate, exit pressure, and wall cooling condition held fixed at steady values, there is a very strong sensitivity to high amplitude pressure fluctuations and flow rate pulsations at the condenser inlet. This sensitivity often significantly alters the condenser mean inlet pressure, pressure drop, local heat transfer rates (>200% increase at certain locations), and the condensing flow morphology. These effects are representative of fluctuations/pulsations that are typically encountered in applications but are either not accounted for or are not detected. The effects of imposed fluctuations/pulsations, as opposed to cases involving negligible imposed fluctuations/pulsations, are dependent on the amplitude and the frequency content of the imposed fluctuations and this is discussed in a separate paper. A significant upstream annular regime portion of the reported shear/pressure driven fully condensing flows operate under conditions where the laboratory??s transverse gravity effects are negligible and, therefore, the identified sensitivity phenomenon is highly relevant to zero- or micro-gravity conditions. The micro-gravity relevance of this sensitivity for the annular regime phenomenon is currently also being demonstrated with the help of computations and simulations.     

Effect of a non-constant magnetic field on natural convection in a horizontal porous layer heated from the bottom

An analytical and numerical investigation is conducted to study the effect of an electromagnetic field on natural convection in a horizontal shallow porous cavity filled with an electrically conducting fluid. The magnetic field is assumed to be induced by two long wires, carrying current, parallel to the horizontal boundaries of the system. A uniform heat flux is applied to the horizontal walls of the layer while the vertical walls are adiabatic. The governing parameters of the problem under study are the thermal Rayleigh number, Ra, Hartmann number, Ha, position of the electrical wires, d, current intensity ratio, r, and aspect ratio of cavity, A. An analytical solution, valid for a shallow layer (A ? 1), is derived on the basis of the parallel flow approximation. The critical Rayleigh number, Ra _c , for the onset of motion is derived in closed form in terms of the parameters of the problem. For finite-amplitude convection the heat and flow characteristics predicted by the analytical model are found to agree well with a numerical study of the full governing equations.     

Heat transfer coefficient during two-phase flow boiling of HFC-134a

This paper reports a study of the evaporation of HFC-134a inside smooth, horizontal tubes. Tests were performed with the pure refrigerant and with oil-refrigerant mixtures. The heat flux was varied from 2 to 10 kW m⁻². The inner diameter of the tubes was 12 mm. Two evaporators were used, 4 and 10 m long, and the oil content was varied from 0 to 2.5 mass percentage (synthetic oil, EXP-0275). Oil-free HFC-134a had a higher heat transfer coefficient than HCFC-22 at the same heat and mass fluxes. The effect of oil in the refrigerant is dependent on the heat flux. At 2 and 4 kW m⁻² the heat transfer coefficient had a maximum value for an oil content of around 0.5 mass percentage; no increase is registered for a heat flux of 6 kW m⁻². The heat transfer coefficients for the pure refrigerant were also compared with two existing correlations. The measured heat transfer coefficients averaged over the evaporator deviate less than 40% from the correlation according to Pierre. The heat transfer coefficients at the short evaporator lie within 20%. The correlation given by Jung overestimates the heat transfer coefficient by approximately 50%.     

Effect of thermal conductivity ratio on flow features and convective heat transfer

In this paper, we numerically investigate the steady laminar natural convection in a water-filled 2D enclosure containing a rectangular conducting body. Computations are performed for a wide range of dimensionless parameters including the Rayleigh number Ra, the thermal conductivity ratio k, and the location of the inner body δ, while the value of the Prandtl number is maintained constant at 6.8. This simulation spans four decades of Rayleigh number, Ra, from 103 to 106. The influence of these various dimensionless parameters on the flow behavior is investigated. Correlations of the averaged Nusselt numbers are obtained as a function of two parameters (Ra and δ) for each working thermal conductivity ratio. The results indicate that the heat transfer rate through the enclosure can be controlled by the position of the rectangular body. A comparative study was also carried out between a horizontal and a vertical conducting shape inside the enclosure. It is proved that a vertical position leads to a better heat transfer compared to the horizontal case.     

The Process of Heat Transfer by Natural Convection in a Square Enclosure the Temperature of One of Whose Walls Varies by Sine Law

The unsteady-state natural convection in a square enclosure arranged at an angle @alpha (0 to 90@deg) to the horizontal is numerically investigated. One of the sides of the enclosure is maintained at a constant temperature, and the other, opposite side, at a temperature, which varies by the sine law and, on the average during a period, is lower than that of the side with constant temperature. The other two sides of the enclosure are taken to be adiabatic. Numerical solutions are obtained for the Grashof number of 5@times10⁵ and the Prandtl number equal to unity, in a wide range of variation of the oscillation frequency of the temperature of one of the walls of the square enclosure. The results are obtained using the control volume approach on the basis of the SIMPLER algorithm.     

Boiling heat transfer from a horizontal flat plate in a pool of liquid hydrogen

Heat transfer from a flat plate facing upward immersed in a liquid hydrogen pool was measured for the pressures from atmospheric to 1.1 MPa. The flat plate heater used was 10 mm in width, 100 mm in length and 0.1 mm in thickness. Critical heat fluxes (CHFs) in saturated boiling increased with the increase in pressure up to around 0.3 MPa and then decreased with further pressure increase. The CHFs under subcooled condition at each pressure increased with the increase in sub-cooling. Discussions were made on the experimental results by comparing with those of the other cryogenic liquids and also the Kutateladze’s equations under saturated and subcooled conditions. The experimental CHFs were much smaller than the Kutateladze’s equation for higher pressure up to critical. The heater surface temperature was found to reach the critical temperature before the occurrence of hydrodynamic instability and jump to the film boiling regime at the lower heat flux in the higher pressure range. It was suggested that the CHFs are determined not by the heat flux but by the temperature in the higher pressure range.     

CO_2微通道气冷器流量分配和换热特性的数值模拟及验证

本文建立了CO_2微通道气冷器集流管和微通道扁管两部分的物理模型并进行网格划分,模拟研究了扁管插入集流管深度f分别为4、5、6 mm和入口管在集流管1/6、1/2位置处对质量流量分配的影响,实验验证了CO_2微通道气冷器扁管壁面温度分布。结果表明:当f为4 mm、入口管位于集流管1/6处时,质量流量分配最均匀,此时不均匀度为0.4×10~(-3);模拟扁管内CO_2换热特性发现随着CO_2质量流量的增加,扁管换热量增加,流量由2.3 kg/h增至2.5 kg/h,换热量提高了21.4%;当质量流量一定时,CO_2的出口温度随着CO_2入口温度的升高而升高,在不同CO_2入口温度条件下,微通道扁管壁面温度实验值与模拟值误差在10%以内,验证了模拟的准确性。