Flow Structure and Heat Transfer Between Two Disks Rotating Independently

In the present paper, fluid flow and convective heat transfer between two co-axial disks rotating independently are dealt with mainly based on the author's recent research on that topic. Three rotational modes, i.e. co-rotation, rotor-stator, and counter-rotation, are considered. Theory of rotating non-isothermal fluids with the presence of disk rotation and thermal effects is addressed. Rotational buoyancy effects on the flow structure development are highlighted. Results of flow visualization and heat transfer measurements are discussed to explore the thermal flow mechanisms involved in the two-disk flows at various rotational and geometric conditions. Potential issues open to the future investigation are also proposed.     

Convective heat transfer induced by fluid oscillations

Numerical simulation was conducted on the heat transfer process in a two-dimensional flow field imposed by a sinusoidal oscillation of the fluid. When gravitational force was ignored, the heat transfer at the wall was similar to forced-flow turbulent heat transfer. On the other hand, considering gravity, two modes of mixed convection heat transfer were observed depending on the thermal boundary conditions. For high-viscosity fluids, a bifurcation of the flow pattern appeared, which was mainly determined by the oscillation amplitude. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 26 (1): 39–53, 1997     

Flow field and heat transfer analysis of local structure for regenerative cooling panel

Local structure of cooling panel has great effects on the heat transfer in the cooling channel for the scramjet. The problems of flow dead area and mass flow rate non-uniform distribution caused by the local structure effect the cooling effectiveness in the channel seriously. Numerical simulation to the flow field of scramjet cold panel with four different fuel injection island structures respectively has been carried out using the CFD commercial software-CFX in this research. The results reveal that flow dead area has been eliminated and flow field has been improved for the optimized structure. Furthermore, local resistance loss has been decreased and the mass flow rate non-uniform distribution in the channel has been reduced. Based on the optimized results, some suggestions about the local design of cooling panel have been proposed in this research.     

Flow and heat transfer over shallow cavities

The flow over a shallow cavity heated with constant heat flux form the bottom side has been studied experimentally and numerically. In the experimental part, pressure coefficient and Nusselt number have been measured along the cavity floor. In the numerical simulation, the conservation equations of mass, momentum, and energy have been solved. The standard k–? turbulence model is used to account for the turbulent fluctuations. In both experimental and numerical part, the effect of changing cavity aspect ratio and Reynolds number has been studied. It is found that, a single elongated eddy has been formed for aspect ratio lower than 7. As the aspect ratio increases the flow impinges with the cavity floor creating two eddies, one beside the upstream cavity side and the second beside the downstream cavity side. Local Nusselt number along the cavity floor is affected mainly by the flow structure inside the cavity and the average Nusselt number has increased with increasing the aspect ratio up to 10 and with increasing Reynolds number. A correlation for the average Nusselt number, as a function of Reynolds number and cavity aspect ratio, is developed.     

The effect of external vorticity on stagnation-point heat transfer at high prandtl number

The strong sensitivity of stagnation-point heat transfer to the presence of free-stream turbulence has been attributed to vorticity amplification by stretching of vortex filaments. A mathematical model proposed by Sutera, Maeder and Kestin is examined for the case of very high Prandtl number (Pr). Asymptotic solutions are obtained for the limit Pr → ∞. These show that the heat-transfer sensitivity continues to increase with Prandtl number. Approximately 95 per cent of the asymptotic value of the stagnation-point heat-transfer coefficient is attained at Pr = 100.     

Influence of packed honeycomb ceramic on heat extraction rate of packed bed embedded heat exchanger and heat transfer modes in heat transfer process

Under the condition that the transient oxidation heat extraction process of coal mine ventilation air methane (VAM) is equivalent to a series of steady state process, the steady state heat extraction experiment platform is built. The influence of the honeycomb ceramic packed in heat extraction zone and its two-side space on heat extraction rate and heat transfer modes is investigated. The experimental results show that the honeycomb ceramic packed in heat extraction zone two-side space can always strengthen heat extract ion of heat exchanger by increasing gas physical flow velocity in bed and radiation heat exchanging area and disturbing heat exchanger leeward side flow field. The contradictory dual characteristic of the influence of the honeycomb ceramic packed in heat extraction zone on heat exchanger heat extraction rate determines that the honeycomb ceramic has no great influence on heat extraction rate and doesn't always strengthen heat exchanger heat extraction. Contribution of heat transfer modes on packed bed embedded heat exchanger heat extraction is investigated using the method of coating heat exchanger outer surface silver; the experimental result shows that 55% contribution of packed bed embedded heat exchanger heat extraction rate is from radiation when gas mass flow rate is 0.15 kg·s^− 1·m^− 2 and its temperature is 1113 k; with the gas temperature being increased further, radiation will become the main way of packed bed embedded heat exchanger heat extraction.     

Impingement of an impact jet onto a spherical cavity. Flow structure and heat transfer

An experimental study of flow characteristics and heat transfer for jet impingement cooling of obstacles in the form of single spherical cavities is reported. The distributions of flow velocities between the nozzle and the obstacle, and also the fields of pressure and heat-transfer coefficients inside the cavity were measured. It is found that, at a value of depth the cavity generates the large-scale toroidal vortex, essentially influencing on the heat transfer. The cavity flow becomes unstable, exhibiting low-frequency pulsations of local heat fluxes. In the examined ranges of Reynolds numbers, Re = (1.2–5.8)10⁴, and cavity depths (equal to or smaller than 0.5D_c) the local heat-transfer intensity in the cavity is lower than that on a flat obstacle; yet, this reduction is almost fully compensated by increased area of the heat-exchanging surface.     

Experimental study on Marangoni effect induced by heat and mass transfer

Surface tension gradients along a fluid–fluid interface provoke strong convective activity, such effect is called Marangoni effect. The Marangoni effect might be induced by heat and mass transfer. In both cases, instabilities develop at the interface. The importance of Marangoni effect has led to many investigations over several years [1], [2], [3], [4], [5], [6] [L. Scriven and C. Sternling, Nature, 187, pp. 186–188 (1960), H. Lu, Y. Yang and J. Maa, Ind. Eng. Chem. Res., 35, pp. 1921–1928 (1996), V. Kaminsky, A. Vyaz'min, N. Kulov and V. Dil'man, Chem. Eng. Sci., 53, No. 19 (1998), P. Lyford, H. Pratt, F. Greiser and D. Shallcross, Can. J. Chem. Eng., 76 (1998a), P. Lyford, H. Pratt, F. Greiser and D. Shallcross, Can. J. Chem. Eng., 76 (1998b), H. Lu, Y. Yang and J. Maa, Ind. Eng. Chem. Res., 36, pp. 474–482 (1997)]. The aim of this work is to study experimentally the Marangoni effect induced by heat and mass transfer, and to determine the Marangoni number (Ma) in both cases. Four aliphatic alcohol were studied, from C1 to C4. In the experiments, interfacial instabilities were observed and Ma was calculated for different temperatures and pressures. It was observed that Ma decreases as the carbon number atoms (n) increase, this behavior can be explained through the changes of some properties (surface tension, diffusivity) as heat and mass transfer occurred. In heat transfer experiments, the highest temperature gradients were reached for methanol, the Bond number (Bd) was also calculated and it was found that the natural convection predominated. In mass transfer experiments, CO₂ was used as gas phase, and it was observed that as pressure increases the Ma increases, it might be explained by the decrease of CO₂ diffusion in hydrocarbons as pressure increases. In conclusion, the Marangoni effect was observed for aliphatic alcohol, the influence of temperature and pressure was also observed, and finally, the Marangoni number decreases as n increases.     

Flow structure and heat transfer in a lower half heated and upper half cooled rectangular enclosure

This paper presents an experimental and numerical investigation on the natural convection in a rectangular enclosure with lower-half-heated, upper-half-cooled sidewalls. The flow structure contains thin wall-layers and large central vortical circulating zones. The mixing process and the fluid paths caused by the wall-layer head-on collisions and stream–stream interaction are fundamental to the development of the heat and fluid transport mechanisms. The understanding of the flow structures and their mutual interaction provides a guide leading to the optimization of the autoclave internal architecture and to practical suggestions for the design strategy of crystal growth vessels.     

新型强化换热管管外流动与换热特性

针对新型强化换热管的几何特征,采用三维数值模拟方法,分别从速度—压力场协同性、综合评价因子ε、速度—温度场协同角三个方面对扭曲管、波节管、波纹管开展研究。对波纹管的实验测量结果与数值模拟结果最大相对偏差为2.35%。数值模拟结果表明,纹管管外Re从569~7 399变化,总压降从3.78增长到317.30Pa;扭曲管Re从508~7 622变化,总压降从0.84增大到62.50Pa;波节管Re为580~7 550变化,总压降从1.29增长到110.00Pa,波纹管阻力相比波节管最大提高186.7%,相比于扭曲管提高408.4%,Re<4 500时扭曲管Nu数比波纹管最大提高13.30%,比波节管最大提高6.97%,Re>4 500时波节管换热性能逐渐超越扭曲管以及波纹管,在计算范围内Nu比扭曲管最大提高5.17%,比波纹管最大提高5.40%;波节管的周期性波峰对于强化换热和减阻作用显著;波纹管的压降、强化换热表现均弱于波节管和扭曲管。根据计算结果拟合出Nu数与Re、Pr的实验关联式,为管壳式换热器设计提供一定的理论和依据。     

Flow boiling heat transfer in circulating fluidized bed

In order to enhance heat transfer and mitigate contamination in the boiling processes, a new type of vapor-liquid-solid (3-phase) circulating fluidized bed boiling system has been designed, combining a circulating fluidized bed with boiling heat transfer. Experimental results show an enhancement of the boiling curve. Flow visualization studies concerning flow hydrodynamics within the riser column are also conducted whose results are presented and discussed.     

Flow boiling heat transfer in circulating fluidized bed

In order to enhance heat transfer and mitigate contamination in the boiling processes, a new type of vapor-liquid-solid (3-phase) circulating fluidized bed boiling system has been designed, combining a circulating fluidized bed with boiling heat transfer. Experimental results show an enhancement of the boiling curve. Flow visualization studies concerning flow hydrodynamics within the riser column are also conducted whose results are presented and discussed.     

Flow boiling heat transfer characteristics of wide-boiling mixtures

Heat transfer coefficients in horizontal flow boiling of the wide-boiling binary mixture of C₂F₆/C₂H₂F₄ and of the ternary mixture C₂F₆/SF₆/C₂H₂F₄ are measured. The experiments are carried out with the condition of uniform inner wall temperature at the tube perimeter. Results of the two boiling regions—forced convective evaporation and nucleate boiling—are obtained. In the forced convective evaporation region the results are predicted best using a pure fluid correlation which takes mixture properties into account. In the nucleate flow boiling region a degradation of the heat transfer coefficients occurs. This degradation has to be predicted with a suitable mass transfer controlled model, based on the ideal heat transfer coefficient, provided that a certain heat flux limit is not exceeded. In the region of high heat fluxes an “effective” heat transfer coefficient, based on local total vaporization, is more suitable than the previously cited ideal heat transfer coefficient.     

球窝的流动控制及分离转捩特性研究

流体经过一定的逆压梯度容易发生流动分离从而大大减小流动效率,球窝结构具有良好的流动控制效果,在机翼、航空发动机、汽轮机叶片等工业领域有较大的应用前景。对一典型收缩扩张通道的流动分离转捩状况进行了数值模拟,并提出了一种带有球窝结构的被动控制方法。研究结果表明:球窝结构作为一种被动流动控制方法,布置在具有明显逆压梯度的通道上能起到良好的流动控制作用,并且能诱导层流边界层提前向湍流边界层转捩,抑制了通道中的流动分离,减小分离泡的尺度,其中球窝的布置位置以及流动Re均对球窝的控制作用有重要影响。球窝的引入还将减小通道的总压损失系数,起到了流动减阻的效果,表明球窝结构是一种较优的流动控制方法。     

Flow and heat transfer over an unsteady stretching surface with non-uniform heat source

The non-uniform heat source/sink effect on the flow and heat transfer from an unsteady stretching sheet through a quiescent fluid medium extending to infinity is studied. The boundary layer equations are transformed by using similarity analysis to be a set of ordinary differential equations containing three parameters: unsteadiness parameter (S), space-dependent parameter (A?) and temperature-dependent parameter (B?) for heat source/sink. The velocity and temperature fields are solved using the Chebyshev finite difference method (ChFD). Results showed that the heat transfer rate, − θ′(0) and the skin friction, − f″(0) increase as the unsteadiness parameter increases whereas decrease as the space-dependent and temperature-dependent parameters for heat source/sink increase.     

Study on heat transfer characteristics of reservoir embedded loop heat pipe (Influence of condenser cooling method on heat transfer characteristics)

As heat generation in satellites increases, securing sufficient radiator panel area is an important problem. Deployable radiators, whose radiator panels are deployed post‐launch in space to increase the effective radiator panel area of the satellite, is becoming an important thermal control technology. A reservoir embedded loop heat pipe (RELHP) is applied to the deployable radiator for a thermal transport device. This paper presents the heat transport dynamic characteristics of a RELHP using a radiant cooling condenser and liquid forced convection cooling condenser by an experimental study. It was found that heat leak into the liquid line, flexible line, and reservoir increases the length of the sub‐cooling region in the condenser. In the case of the radiant cooling condenser, the sub‐cooling region length is shorter than that of a liquid forced convection cooling condenser. Furthermore, vapor temperature is mainly decided by the radiation capacity of the radiator panel, because liquid temperature returned into the evaporator rises with an increase in radiator panel temperature. In addition, time length from start‐up until steady state is greater than the liquid forced convection cooling condenser case, because the radiator panel has a large heat capacity. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20229     

Flow structure and heat transfer mechanism of laminar vortex rolls driven by gravity currents between parallel plates of nonuniform wall temperature

Laminar vortex rolls which occur between horizontal parallel plates, of which the temperature is not uniform in the lateral direction normal to the forced main flow, are investigated by means of a flow-visualization technique and a 3D steady-state numerical calculation. It was found that the vortex rolls were driven by the horizontal gravity currents, which were caused by density difference in the lateral direction, and that the generation mechanism of the vortex rolls could be classified into four types. The effects of vortex rolls on the local Nusselt number were also investigated. © 1999 Scripta Technica, Heat Trans Asian Res, 28(7): 606–619, 1999     

Flow and impingement cooling heat transfer along triangular rib-roughened walls

Experiments are performed to study slot air jet impingement cooling flow and the heat transfer along triangular rib-roughened walls. Both flow visualization and local heat transfer measurements along the ribbed wall are made. The effect of different rib protrusions (heights) on the impinging flow and heat transfer along the wall is studied, which is achieved by using different sizes of nozzles. Two different ribbed walls with different rib pitches are selected which have a rib pitch-to-height ratio of 2 and 4, respectively. The widely opened cavity between neighboring ribs make more intense transport of momentum between the wall jet and cavity flow so that recirculation cell in the cavity is hardly observed. This leads to a higher heat transfer around the cavity wall than in the case with rectangular ribs. However, in the region of laminar wall jet, a number of air bubbles enclosing the cavities are formed which prevent penetration of the wall jet into the cavities. This leads to a significant reduction in the heat transfer. The geometric shape of the triangular ribs is more effective in rebounding the wall jet away from the wall than in the case with rectangular ribs. The rebound of the jet away from the wall causes a significant reduction in the heat transfer. A comparison and correlations of the stagnating point Nusselt number under different conditions are presented and discussed. During the experiments, the Reynolds number varies from 2500 to 11,000, the slot width-to-rib height ratio from 1.17 to 6.67, and nozzle-to-plate spacing from 2 to 16.     

Flow and heat transfer in compact offset strip fin surfaces

Experimental studies of air-side heat transfer and pressure drop characteristics of offset strip fins and flat tube heat exchangers were performed. A series of tests were conducted for 9 heat exchangers with different fin space, fin height, fin strip length and flow length, at a constant tube-side water flow rate of 2.5 m³/h. The characteristics of the heat transfer and pressure drop of different fin space, fin height and fin length were analyzed and compared. The curves of the heat transfer coefficients vs. the pumping power per unit frontal area were then plotted. Moreover, the enhanced heat transfer mechanism of offset strip fins was analyzed using field synergy theory. The results showed that fin length and flow length have more obviously effect on the thermal hydraulic characteristics of offset strip fins. __________ Translated from Journal of Shanghai Jiaotong University, 2007, 41(3): 366–369, 375 [译自: 上海交通大学学报]     

Flow and heat transfer in compact offset strip fin surfaces

Experimental studies of air-side heat transfer and pressure drop characteristics of offset strip fins and flat tube heat exchangers were performed. A series of tests were conducted for 9 heat exchangers with different fin space, fi