EXPRIMENTAL STUDY ON EFFECTS OF A SINGLE POROUS-TYPE ROUGHNESS ELEMENT IN A PARALLEL-PLATE DUCT

Experiments were carried out to examine the effects of a single porous-type roughness element on the insulated wall opposite the smooth heated plate on the heat transfer. The local heat transfer and drag coefficients depend on the porous diameter and the porosity. The local heat transfer coefficient takes a peak, PI, under the porous element in laminar flow. On the other hand, in turbulent flow, it takes two peaks, PI and P2, under and after the element, respectively. The position of peak P2 varies with the height of the element and the Reynolds number. The drag coefficient of the porous element is lower than that of the solid element. According to thermal performance at constant pumping power, this kind of element should be used in laminar flow. In addition, it is estimated that the porous element should be utilized in the composite effects (the turbulence increase and the thermal radiation shielding effect) of heat transfer in order to apply the element effectively.     

Infrared thermography as a tool for thermal surface flow visualization

Infrared (IR) thermography is a two-dimensional, non-contact technique of temperature measurement which can be usefully exploited in a vast variety of heat transfer industrial applications as well as research fields. The present work focuses attention on thermal surface flow visualizations of several types of fluid flow studied by means of the IR imaging system and in particular: the flow over a delta wing at angle of attack; the flow generated by a disk rotating in still air; air jets impinging on a flat wall; the flow inside a 180deg turn in astatic channel with, or without, turbulence promoters; the flow inside a 180deg turn in arotating square channel. Each flow visualization is illustrated through thermographic images and/or Nusselt number maps. The emphasis is on the capability of the infrared system to study: laminar-to-turbulent transition and location of primary and secondary vortices over the delta wing at angle of attack; the spiral vortical structure developing at transition over the disk; azimuthal structures arising for certain jet conditions; the influence of the channel aspect ratio (width to height ratio) on the heat transfer coefficient distribution along the 180deg turn, as well as the influence of ribs, in the case ofstatic channel; the influence of rotation for the rotating channel.     

Heat transfer in tubes with periodic draft under supercritical conditions

The temperature mode of a wall was studied experimentally for sub-and supercritical pressures of water in tubes with turbulence stimulators; these data were compared with similar results obtained for a smooth tube. An increase in heat transfer inside a turbulizing tube was revealed for a single-and two-phase water flows. Correlation dependencies for determination of heat transfer coefficient in a single-phase flow inside a tube with turbulence stimulators were obtained.     

Three-dimensional direct numerical simulation of turbulent channel flow catalytic combustion of hydrogen over platinum

The turbulent catalytic combustion of a fuel-lean hydrogen/air mixture (equivalence ratio ? = 0.24) was investigated by means of three-dimensional direct numerical simulation (DNS) in a platinum-coated plane channel with a prescribed wall temperature of 960 K and an incoming Reynolds number, based on the channel height, of 5700. Heat transfer from the hot catalytic walls laminarized the flow, as manifested by the progressive suppression of the high vorticity components of the flow aligned parallel to the channel walls at increasing streamwise distances. The impact of turbulence suppression on the mass transfer towards or away from the catalytic wall was subsequently assessed. Far upstream where high turbulence fluctuations persisted, the instantaneous local transverse gradient of the limiting hydrogen reactant (a quantity proportional to the catalytic reaction rate) as well as the instantaneous hydrogen concentration at the wall exhibited strong fluctuations by up to 300%, a result of finite-rate chemistry induced by the high inrush events towards the catalytic walls. Fourier analysis of the reaction rate fluctuations yielded peak frequencies of less than 1 kHz, values comparable to the thermal response frequencies of typical materials in commercial catalytic geometries. This has direct implications on the thermal stress of the reactor walls as well as on the decoupling between flow and solid thermal modeling currently used in practical catalytic reactors. Far downstream, the dampening of turbulence resulted in weaker hydrogen concentration fluctuations with nearly symmetric distributions. Finally, computed transverse turbulent species fluxes indicated inherent weaknesses of near-wall turbulence models in describing turbulent transport of species with disparate molecular diffusivities.     

Experimental study of heat transfer characteristics in oscillating fluid flow in tube

In the present work, an experimental investigation of heat transfer enhancement parameters of the oscillating flow heat exchanger under different frequencies, tidal displacement, and heat fluxes is carried out. The effect of different parameters on experimental effective thermal conductivity and convective heat transfer coefficient in cooling region is studied using correlations given by different researchers. A correlation for experimental effective thermal conductivity in terms of S²√ω is derived based on experimental data. This correlation is useful for predicting the optimum value of S²√ω before onset of turbulence.     

FORCED-CONVECTION HEAT AND MASS TRANSFER FROM COMPLEX SURFACES

Abstract

Heat and mass transfer rates from complex surfaces to a turbulent channel flow were measured using an infrared imaging system and the naphthalene sublimation technique, respectively. The surfaces are composed of spherical particles embedded either in a layer of thermally conducting, nonevaporating liquid or in an isothermal layer of subliming naphthalene. The experimental results indicate that, in general, the surface heat and mass transfer coefficients vary as the surface roughness increases, whereas the surface heat transfer coefficient changes as the solid-to-liquid thermal conductivity ratio is varied. Mass transfer rates exhibit less sensitivity to variations in the naphthalene height for surfaces composed of smaller particles, and heat transfer rates from surfaces of smaller particles remain fairly constant as the liquid level and thermal conductivity ratios are varied. The results are discussed relative to drying of partially wetted surfaces with surface complexity induced by the presence of droplets upon an impermeable substrate or a receding moisture front in a bed of granular material.     

METHOD AND SOME RESULTS ON EXPERIMENTAL STUDY OF UNSTEADY TURBULENT HEAT TRANSFER

Abstract

An experimental setup is described for studying unsteady heat transfer where both the heat generation rate in the channel wall and the fluid flow rate vary in time. For measuring the surface temperature and the wall heat flux, the temperature field in the thermal sublayer is used. The results of the first experiments carried out for a sudden change in channel wall heat generation are discussed. Unsteadiness rates much greater than those in previous work known to the authors were achieved     

BOILING HEAT TRANSFER OF WATER AND R-11 ON HORIZONTALLY SMOOTH AND ENHANCED TUBES ENCLOSED BY A CONCENTRIC OUTER TUBE WITH TWO HORIZONTAL SLOTS

An experimental investigation was carried out on the boiling heat transfer characteristics of water and R-11 on the outside of a horizontal heated tube in narrow spaces. Two kinds of heat transfer surfaces (roll-worked and smooth surfaces) were tested. The test section consisted of a narrow annular space formed by enclosing the heated tube in an isolated concentric outer tube with two horizontal slats on the top and bottom. The nucleate boiling heat transfer characteristics were investigated experimentally at atmospheric pressure. The experimental results indicated that a single roll-worked tube in bulk liquid showed better boiling heat transfer than a single smooth tube. In the narrow spaces, the boiling heat transfer coefficients for the smooth tube were considerably enhanced when the gap size was so selected as to take an optimum value. There was no clear optimum gap size for heat transfer enhancement for the roll-worked tube in the narrow spaces. Enhancement of boiling heat transfer in the narrow spaces for the roll-worked tube was not clearly observed in this experiment. Finally, the critical heat flux (CHF) for boiling heat transfer in narrow spaces can be predicted by using a proposed CHF correlation.     

An Experimental and Theoretical Investigation into the Thermal Performance Characteristics of a Staggered Vertical Pin Fin Array Heat Sink with Assisting Mixed Convection in External and In-Duct Flow Configurations

A theoretical and experimental study was carried out on the thermal performance of a pin fin array heat sink. A theoretical model was utilized based on the success of prior research that has the capability of predicting the influence of various geometrical, thermal, and flow parameters on the effective thermal resistance of the heat sink. An experimental investigation was carried out for measuring the thermal performance of the heat sink, and the overall convective heat transfer coefficient for the fin bundle, including the thermal and flow bundle effect. Utilizing the new empirical correlations, theoretical predictions were made and experimentally validated for a wide range of parameters for combined forced and natural convection in the assisting flow configuration. Both the theoretical model and experimental data indicated the existence of optimal fin spacing.     

纳米流体对流换热系数增大机理

纳米流体流动换热能力优于传统流体介质.研究了纳米流体热物性的提升和热散射对其对流换热系数的影响.结果表明,纳米颗粒的加入,优化了介质的热物性,增大了导热系数,强化了纳米流体内颗粒、流体以及流道管壁碰撞和相互作用,同时加强了流体的混合脉动和湍流,从而增大了对流换热系数. 关键词： 纳米流体 换热系数 热散射     

Experimental Investigation of Mixed Convection in a Channel With an Open Cavity

Mixed convection in an open cavity with a heated wall bounded by a horizontally unheated plate is investigated experimentally. The cavity has the heated wall on the inflow side. Mixed convection fluid flow and heat transfer within the cavity is governed by the buoyancy parameter, Richardson number (Ri), and Reynolds number (Re). The results are reported in terms of wall temperature profiles of the heated wall and flow visualization for Re = 100 and 1000, Ri in the range 30–110 (for Re = 1000) and 2800–8700 (for Re = 100), the ratio of the length to the height of cavity (L/D) is in the range 0.5–1.5, and the ratio of the channel height to cavity height (H/D) is in the range of 0.5 and 1.0. The present results show that the maximum dimensional temperature rise values decrease as the Reynolds and the Richardson numbers decrease. The flow visualization points out that for Re = 1000 there are two nearly distinct fluid motions: a parallel forced flow in the channel and a recirculation flow inside the cavity. For Re = 100 the effect of a stronger buoyancy determines a penetration of thermal plume from the heated plate wall into the upper channel. Nusselt numbers increase when L/D increase in the considered range of Richardson numbers.     

A numerical analysis on flow field and heat transfer by interaction between a pair of vortices in rectangular channel flow

This paper is a numerical study of the effect of flow field and heat transfer created by interactions between a pair of vortices generated by a vortex generator in a rectangular channel flow. In order to analyze the vortices produced by the vortex generator, the pseudo-compressibility method is introduced into the Navier–Strokes (NS) equation of a three-dimensional unsteady, incompressible viscous flow. A two-layer k–ε turbulence model is used on the flat plate three-dimensional turbulence boundary to predict the turbulence characteristics of the vortices. The computational results accurately predict the vortex characteristics, which are related to Reynolds stress, turbulent kinetic energy, and flow field. Also, in the prediction of thermal boundary layers, skin friction characteristics, and heat transfers, the present results are reasonably close to the experimental results obtained by other researchers.     

Experimental characterization of the convective heat transfer in a vortex-wall interaction

The development of turbulence models and wall laws for the numerical simulation of flows in complex geometries requires a detailed experimental analysis of turbulence and of the phenomena that appear in turbulent boundary layers. There is a strong need to develop new measurement systems allowing the determination of unsteady wall heat transfer coefficients. In order to improve the knowledge of the unsteady phenomena occurring in perturbed boundary layers, a fundamental study is conducted on the interaction of a single vortex with a flat plate. An experimental methodology using a specific thermal sensor whose surface temperature is measured by an infrared thermography system is presented. It allows the characterization of the unsteady convective heat transfer coefficient whose evolution is compared with the fluctuations of the wall friction coefficient, calculated from velocity profiles measured by laser Doppler velocimetry.     

Convective Heat Transfer of a Cu/Water Nanofluid Flowing Through a Circular Tube

Abstract

Fluids in which nanometer-sized solid particles are suspended are called nanofluids. These fluids can be employed to increase the heat transfer rate in various applications. In this study, the convective heat transfer for Cu/water nanofluid through a circular tube was experimentally investigated. The flow was laminar, and constant wall temperature was used as thermal boundary condition. The Nusselt number of nanofluids for different nanoparticle concentrations, as well as various Peclet numbers, was obtained. Also, the rheological properties of the nanofluid for different volume fractions of nanoparticles were measured and compared with theoretical models. The results show that the heat transfer coefficient is enhanced by increasing the nanoparticle concentrations as well as the Peclet number.     

斜翅型冷凝强化换热管传热性能的实验研究

对一种斜翅型外翅片带内螺纹的冷凝强化换热管进行传热性能的实验研究。管外冷凝换热的制冷剂为R134a,管内对流换热的介质为水。分别在定热流密度与定水流速的条件下进行一系列工况的实验,得到相应的实验数据。在定热流密度条件下,利用Wilson图解法得到管内的换热系数数据及相应的计算关联式。在定水流速的条件下,利用分离方法得到管外冷凝换热系数数据及相应的计算关联式。将强化管换热系数数据与光管换热系数的理论计算值进行了比较,结果表明:冷凝强化换热管管内对流换热的强化倍率为2.4,管外凝结换热系数随壁面过冷度的增加而增大,管外凝结换热的强化倍率为:1.78～3.92。     

Methane steam reforming in an annular microchannel with Rh/Al2O3 catalyst

Regularities of methane conversion in the presence of water steam were obtained experimentally while activating chemical conversions on the inner convex wall of an annular microchannel. The steam methane reforming was done on the Rh/Al₂O₃ nanocatalyst with the heat applied through the microchannel gap from the outer wall. Concentrations of the products of chemical reactions in the outlet gas mixture are measured at different temperatures of the outer microchannel wall. The range of channel wall temperatures at which the ratio of hydrogen and carbon oxide in the outlet mixture grows substantially is determined. Data on the composition of methane conversion products for the ratio H₂O/CH₄ = 1.77 and the activation energy of methane steam reforming at reactor outer wall temperatures of up to 880°C are obtained. The effect of the radiation heat exchange and the external diffuse limitation on the rate of chemical conversions in methane steam reforming in an annular microchannel with external heat supply is determined.     

Heat transfer at a laminar free convection and separated flow past a rib in a vertical channel with isothermal walls

The results of numerical computations of a free laminar convection and heat transfer between two parallel isothermal plates in the presence of a single rib on the channel surface are presented. The investigations have been conducted for a channel with the aspect ratio AR = L/w = 10, where L is the channel height, and w is the distance between the plates. An infinitely thin adiabatic rib was located on one of the channel walls in the middle of its height. The relative rib height l/w was varied in the range 0÷0.8. The wall temperature was higher than the ambient temperature, and the Rayleigh number was varied in the range Ra = 10²÷10⁵. The main attention has been paid to the study of the influence of the rib height and the Rayleigh number on local and integral heat transfer and the Reynolds number in the channel (the convective thrust). A fundamental difference in the heat transfer over the channel height has been shown on the ribbed wall and on a smooth surface. The computational results have been compared with the case of a symmetric distribution of the ribs on the both walls with the integral height equal to a single rib.     

Experimental studies on natural convection of water in a closed-loop vertical annulus

Experimental studies on heat transfer and fluid flow of water in a vertical annulus, circulating through a cold leg forming a closed loop thermo-siphon, have been carried out in this article. The annulus has a radius ratio (outer radius to inner radius) of 1.184 and aspect ratio (length to annular gap) equal to 352. The experiments were conducted for constant heat fluxes of 1, 2.5, 5, 7.5, 10, 12.5, and 15 kW/m². Transient behavior during the heat-up period of the system until the steady-state condition is attained and discussed. Variation in the heat transfer coefficient and Nusselt number along the annulus height represent the developing boundary layer at the entrance and fully developed flow in the remaining length. A large drop in the differential pressure is experienced when the liquid is circulated through the flow meters, which restrict the flow due to their very small passages. Flow restriction causes mass accumulation and rise of pressure at the exit of the annulus. It also causes a decrease in liquid head in the cooling leg. An increase in the heat flux leads to an increase in the heat transfer coefficient and Nusselt number. As a result of the data analysis correlations for the average Nusselt number, Reynolds number and circulation rate have been developed in terms of the heat flux.     

Wall Shear Stress and Heat Transfer of Downward Bubbly Flow at Low Flow Rates of Liquid and Gas

The effect of suppression of turbulence in a downward bubbly flow and its impact on the wall shear stress and heat transfer are discussed. Measurements were carried out for Reynolds numbers Re = 5000–10000, which were calculated from the velocity of the liquid phase and with the gas volumetric flow rate ratio β = 0–0.05. Data on the size of bubbles detaching from the edges of an array of capillaries in a liquid flow are given. The influence of the disperse phase dimensions on the wall shear stress and heat transfer is discussed. It is shown that change in the size of the dispersed phase can lead to both intensification and deterioration of heat transfer as compared with a single-phase flow at constant flow rates of liquid and gas at the channel inlet. The cause of the heat transfer deterioration is “laminarization” of the flow in the near-wall region. An analysis of the spectral power of signals is given.     

Experimental study of heat transfer of dielectric liquid perfluorohexane at flowboiling in a microchannel heat exchanger

Presented are results of an experimental study of local heat transfer characteristics in boiling of the dielectric liquid perfluorohexane under forced convection in a horizontal microchannel heat exchanger. The experiments with a copper microchannel heat exchanger comprising 21 channels with sections of 335 × 930 μm were conducted with a mass velocity of 250 to 1000 kg/m²s and a heat flux through the outer wall of the heat exchanger of 3 to 60 W/cm². The dependence of the local heat transfer coefficient on the heat flux density on the inner wall of the microchannels was established, as well as the critical heat flux. The experimental data are compared with calculations based on known models of heat transfer.