Local measurements in the flow of a steam injector and visualisation

A steam injector is a thermocompression device where the steam drags directly a subcooled liquid and delivers a liquid with an outlet pressure higher than both upstream fluid pressures. The functioning principle of injectors involves sophisticated thermohydraulic processes whose present knowledge is essentially empirical. This work presents an experimental approach of steam injector in order to finely understand and quantify the physical laws driving the flow in the mixing chamber. Local measurements (2D) are carried out on a special device with a rectangular section, where the flow was visualised. These measurements are the void fraction, the static pressure and the static temperature. They permit the calculation of all the variables and a well understanding of the physical phenomena involved in the flow. Especially, we show, in some well-defined zones, the existence of important non-equilibrium kinetic, thermal and thermodynamic phenomena.     

Modeling of hot gas flow through a feed stream within a horizontal pipe

High heat penetration into a feed stream within a horizontal pipe is described mathematically with a gas flow and heat transfer model. Influences of varied factors on the gas flow and heat transfer in porous media are examined for different conditions. The temperature of the packed‐bed particles and the gas velocity distribution curves are obtained for the feeding service at interruption and at regular charge operating conditions. The numerical results show that the thermal effect to the packed‐bed particles by the seepage flow fluid is high only in the position near the gas entrance. The thermal penetration depth tends to increase with the seepage flow velocity and decrease with the feed rate. The operating conditions and the porosity of solid bed have importance effects on the gas velocity and temperature field in the thermal penetration zone. The model results are found to compare favorably with the experimental data available in the literature. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(6): 553–565, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10109     

Local Heat and Mass Transfer for Gas—Solid Two Phase Flow in CFB

ＬｏｃａｌＨｅａｔａｎｄＭａｓｓＴｒａｎｓｆｅｒｆｏｒＧａｓ－ＳｏｌｉｄＴｗｏＰｈａｓｅＦｌｏｗｉｎＣＦＢＦｅｎｇＬｕ；Ｍｉｎｇ－ＨｅｎｇＳｈｉ（Ｄｅｐｔ．ｏｆＰｏｗｅｒＥｎｇ．，ＳｏｕｔｈｅａｓｔＵｎｉｖｅｒｓｉｔｙ，Ｎａｎｊｉｎｇ，２１００１８，...     

Analytical modelling of heat transfer from a single slab in freezing

The present study deals with the experimental and theoretical analysis of the transient heat transfer from an individual slab body to the medium during freezing. In this respect, a new model was developed to determine the heat transfer coefficient and this heat transfer coefficient was used in the computation of the dimensionless theoretical temperature distribution. On the other hand, experimental work was performed to measure the centre temperatures of the individual slab products (viz. dried figs) during freezing at the median temperature of -22°C. In the comparison of the theoretical and experimental temperature distributions, very good agreement was found.     

Calorimeter measurements of a heat pump dehumidifier: Influence of evaporator air flow

The performance characteristics of a Rankine cycle heat pump dehumidifier have been measured in a psychrometric calorimeter which provides close control of the air state. The quality of the measurements was monitored by applying energy and mass balance test criteria. The results show that the influence of the evaporator air flowrate on the moisture extraction rate, and on the energy efficiency of the drier, depends on the relative humidity of the evaporator inlet air stream. When the relative humidity is high the dehumidification efficiency is maximized when the evaporator air flow is maximized, within the range of air flowrates investigated. Below 50% relative humidity the specific moisture extraction rate exhibits an increasingly peaked maximum as a function of the evaporator air flowrate. This maximum moves to lower air flows when the relative humidity is lower.     

Design and fabrication of coaxial surface junction thermocouples for transient heat transfer measurements

Low cost coaxial surface junction thermocouples (CSJTs') have been fabricated in-house and calibrated to measure the transient surface temperature rise within a UNITEN's shock tube wall facility, consisting of K-type coaxial thermocouple elements. The aim of this paper is to explain the design technique of the CSJTs' and the difficulties that have occurred during the fabrication process. The microstructural analysis and the chemical characterization for these types of thermocouples have also been carried out to verify the surface morphology and to qualitatively evaluate the CSJT materials composition. The preliminary testing was performed to demonstrate the performance of these thermocouples to be used for measuring the surface temperatures and heat transfer rates under transient conditions. The preliminary results from shock tube tests have shown that these thermocouples have a time response on the order of microseconds and were suitable for making heat transfer measurements in highly transient conditions. It was concluded that the current construction technique produced gauges that were reliable, reproducible, rugged and inexpensive.     

多孔介质对冷热流体横向射流混合过程热波动的影响

运用流固耦合方法建模,应用FLUENT计算软件平台对填充有多孔介质的T型连接方形管道内冷热流体横向射流混合过程的流动和热传递进行大涡模拟,采用了Smagorinsky-Lilly亚格子模型,获得了瞬时速度和温度分布.结果表明,填充多孔介质能够有效减少T型连接管道中冷热流体横向射流混合的温度和速度波动.固体骨架的导热率较...     

Heat and fluid flow characteristics inside differentially heated square enclosures with single and multiple sliding walls

Fluid flow and heat transfer characteristics of differentially heated lid driven cavities are numerically modeled and analyzed in the present study. One‐, two‐, and four‐sided lid driven cavity configurations are considered with the vertical walls being maintained at different temperatures and the horizontal walls being thermally insulated. Eight different cavity configurations are considered depending on the direction of wall motion. The Prandtl number Pr is taken to be 0.7, the Grashof number is taken to be 10⁴, while two values for the Richardson number Ri are considered, 0.1 and 10. It is found that both the Richardson number and the cavity configuration affect the heat and fluid flow characteristics in the cavity. It is concluded that for Ri=0.1, a four‐sided driven cavity configuration with all walls rotating in the same direction would triple the value of the average Nusselt number at the cold wall when compared to a one‐sided driven cavity configuration. However, for Ri=10, the cavity configuration has minimal effect and all eight cases result in an average Nusselt number value at the cold wall ranging between 1.3 and 1.9. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/htj.20264     

Heat transfer around a circular cylinder in separated air flow

An experimental study has been conducted to determine the heat transfer characteristics around a circular cylinder attached to the separated flow of air shed from a fence. The fence was located vertically to the flow with a height of H = 40 mm. d/H was constant at 0.638, where d is the cylinder diameter of 25.5 mm. X/H were 0.50 and 0.775 and Y/H ranged from 0.525 to 1.50, where X and Y are, respectively, the distances between the axis of the cylinder and the front face of the fence, and the bottom wall of the test section. The Reynolds number based on the cylinder diameter and the velocity of the undisturbed flow ranged from 1.9 × 10⁴ to 6.0 × 10⁴. It was found that the maximum local Nusselt number changes drastically in the vicinity of Y/H = 1.0–1.11 and that the maximum mean Nusselt number occurs in the neighborhood of Y/H = 1.24–1.43 for X/H = 0.50 and 1.3–1.4 for X/H = 0.775. © 1999 Scripta Technica, Heat Trans Asian Res, 28(3): 211–226, 1999     

Convective heat transfer inside a rotating cylinder with an axial air flow

This article presents an experimental identification technique for the convective heat transfer coefficient inside a rotating cylinder with an axial airflow. The method consists in heating the outer face of the cylinder using infrared lamps, and acquiring the evolution of the external surface temperature versus time using an infrared camera. Heat transfer coefficients are identified via three methods. The first one is based on an inverse model, the second one assumes the wall of the cylinder as a thermally thin wall and the third one is based on an analytical method permitting to obtain the temperature field within the whole cylinder. The experiments were carried out for a rotational speed ranging from 4 to 880 rpm corresponding to rotational Reynolds numbers varying from 1.6×10³ to 4.7×10⁵ and an air flow rate varying from 0 to which corresponds to an axial Reynolds numbers ranging from 0 to 3×10⁴. Correlations connecting the Nusselt number to the axial and rotational Reynolds numbers are also proposed.     

Heat Transfer and Fluid Flow over a Single Disk in a Fluid Rotating as a Rigid Body

Laminar heat transfer problem is analyzed for a disk rotating with the angular speed ωin a co-rotating fluid (with the angular speed Ω). The fluid is swirled in accordance with a forced-vortex law, it rotates as a solid body at β= Ω/ω= const. Radial variation of the disk's surface temperature follows a power law. An exact numerical solution of the problem is obtained basing on the self-similar profiles of the local temperature of fluid, its static pressure and velocity components. Numerical computations were done at the Prandtl numbers Pr = 1(?)0.71. It is shown that with increasing βboth radial and tangential components of shear stresses decrease, and to zero value at β= 1. Nusselt number is practically constant at β= 0(?) 0.3 (and even has a point of a maximum in this region); Nu decrease noticeably for larger βvalues.     

Dynamic and thermal study of air flow control by chicanes with inclined upper parts in solar air collectors

A dynamic and thermal simulation for two-dimensional model is developed on air flow and heat transfer control by chicanes in solar air collectors. New chicane form is adopted with two parts: the first is orthogonal to the air flow direction and the second is titled (α=60°). It is apparent that the turbulence created by introducing chicanes, resulting in greater increase in heat transfer inside the dynamic air vein with a rise of 23%. The effect of the variation of the Reynolds number in the range of 100<Re<4500, on the convective heat transfer coefficient, the pressure drop and Nusselt number are analysed and have shown good agreements with the literature results. Therefore, the mass flow rates effect on the velocity magnitude, temperature and the turbulent intensity is analysed. The Reynolds number variation showed a substantial effect on the mechanism of vortex development and separation phenomenon.     

A thermal dispersion model for single phase flow in porous media

A new model for thermal dispersion is proposed based on the character of flow in porous media which relates the dispersion with the velocity and the tortuosity of fluid flow in the pores. Compared with former similar models, this one has fewer adjustable empirical constants, and therefore it is expected to be used more conveniently. The prediction by the present model can successfully reproduce the temperature distributions in the nearby wall region when a single phase fluid flows through a packed channel. The results of Nu versus Re agree well with the experimental results. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(6): 545–552, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10107     

Enhancement of heat transfer from a flat surface in a channel flow by attachment of rectangular blocks

Enhancement of the heat transfer from a flat surface in a channel flow by attachment of rectangular cross‐sectional blocks has been investigated as a function of Reynolds number (Re), arrangement of the blocks with respect to the main flow direction as well as each other, and the numbers (spacing) of the blocks. The channel had a cross‐sectional area of 80×160 mm² (i.e. an aspect (width‐to‐height) ratio of 2). Re, based on the hydraulic diameter of the channel (D_e) and bulk mean velocity (u), changed in the range of 6670–40000. The blocks were positioned both transverse and parallel with respect to the main flow direction. The parallel blocks were arranged in both in‐line and staggered orientation with respect to each other. The effect of the blocks on the flow pressure drop was also measured. The results indicated that the heat transfer could be enhanced or reduced depending on the spacing between the blocks and their positioning and arrangement. For a given pressure drop, the best heat transfer enhancement by the blocks over that from a smooth surface (without blocks) was obtained when the blocks were positioned parallel to the flow and arranged in a staggered manner with respect to each other. Copyright © 2001 John Wiley & Sons, Ltd.     

Heat transfer and fluid flow analysis of an artificially roughened solar air heater: a CFD based investigation

In this paper, the effect of rib (circular sectioned) spacing on average Nusselt number and friction factor in an artificially roughened solar air heater (duct aspect ratio, AR = 5:1) is studied by adopting the computational fluid dynamics (CFD) approach. Numerical solutions are obtained using commercial software ANSYS FLUENT v12.1. The computations based on the finite volume method with the semi-implicit method for pressure-linked equations (SIMPLE) algorithm have been conducted. Circular sectioned transverse ribs are applied at the underside of the top of the duct, i.e., on the absorber plate. The rib-height-to-hydraulic diameter ratio (e/D) is 0.042. The rib-pitch-to-rib-height (P/e) ratios studied are 7.14, 10.71, 14.29 and 17.86. For each rib spacing simulations are executed at six different relevant Reynolds numbers from 3800 to 18000. The thermo-hydraulic performance parameter for P/e = 10.71 is found to be the best for the investigated range of parameters at a Reynolds number of 15000.     

高温空气燃烧系统中陶瓷蓄热体传热特性分析研究

针对小球、圆孔、方格孔、三角孔和正六边形孔蜂窝体等不同几何结构下的陶瓷蓄热体对高温空气燃烧系统的非稳态交替加热和冷却的传热过程的影响进行了理论分析，得出了正方形蜂窝体具有最佳的比表面积和开孔率的结论。建立了陶瓷蓄热体和气体的温度变化微分方程和数值计算的离散方程，并选取实例进行了数值计算，得出了温度变化和传热变化的特性曲线，其与实验测试结果变化规律基本一致。研究结果可以为高温空气燃烧过程中合理有效地控制蓄热体中交替换热过程提供理论依据。     

横管降膜过程中液膜内流动和传热特性分析

横管降膜流动过程中,液膜速度和温度及其分布是影响传热传质的关键因素,由于实验研究方法的局限性,实验研究结果一般只是液膜内各参数的平均值,而液膜内部的速度和温度具体分布特性却很难得到。借助FLUENT软件,利用VOF模型研究了橫管外液膜速度和温度及其分布特性。通过建立三维数理模型,模拟研究了常温常压下,橫管外液膜无相变条件下横管液膜的传热过程,并从边界层的角度解释了液膜波动对传热过程的影响。     

Temperature measurements near a heating surface at high heat fluxes in subcooled pool boiling

In previous papers (Int J Heat Mass Transfer, 2008;50:3481–3489, 2009;52: 814–821), the authors conducted measurements of liquid–vapor structures in the vicinity of a heating surface for subcooled pool boiling on an upward‐facing copper surface by using a conducting probe method. We reported that the macrolayer dryout model is the most appropriate model of the CHF and that the reason why the CHF increases with increasing subcooling is most likely that a thick macrolayer is able to form beneath large vapor masses and the lowest heat flux of the vapor mass region shifts towards the higher heat flux. To develop a mechanistic model of the CHF for subcooled boiling, therefore, it is necessary to elucidate the effects of local subcooling on boiling behaviors in the vicinity of a heating surface. This paper measured local temperatures close to a heating surface using a micro‐thermocouple at high heat fluxes for water boiling on an upward‐facing surface in the 0 to 40 K range of subcooling. A value for the effective subcooling, defined as the local subcooling during the period while vapor masses are being formed was estimated from the detected bottom peaks of the temperature fluctuations. It was established that the effective subcooling adjacent to the surface remains at considerably lower values than the bulk liquid subcooling. This suggests that, from nucleation to coalescence, the subcooling of a bulk liquid has a smaller effect on the behavior of primary bubbles than the extent of the subcooling would appear to suggest. An empirical correlation of the effective subcooling is proposed to provide a step towards quantitative modeling of the CHF for subcooled boiling. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20277     

Fluid flow and heat transfer of mixed convection adjacent to vertical heated plates placed in uniform horizontal flow of air

Experiments have been carried out for mixed convective flows of air adjacent to the vertical heated plates in uniform horizontal forced flows to investigate relationships between the flow and the heat transfer. The experiments cover the ranges of the Reynolds and modified Rayleigh numbers: Re_L = 160 to 2300 and Ra_L^* = 4.3 × 10⁵ to 2.0 × 10⁸. The flow fields over the plates are visualized with particles and smoke. The results show that a stagnation point moves downward away from the center of the plate when the surface heat flux is beyond a critical value. The condition where the stagnation point begins to move is expressed with non‐dimensional parameters as: Gr_L^*/Re_L^2.5 = 0.15. Profiles of measured local heat transfer coefficients are smooth even at the stagnation points in all the cases examined. When buoyancy effect is sufficiently weak, the coefficients agree well with those of the wedge flow. With increasing the surface heat flux, the coefficients are augmented to approach asymptotically the boundary layer solution of natural convection along a vertical heated plate. Finally, forced, mixed, and natural convection regimes are classified by the non‐dimensional parameter (Gr_L^*/Re_L^2.5). © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20256