Numerical analysis of mixed convection heat transfer of a high viscosity fluid in a rectangular tank with rolling motion

A numerical analysis is presented for the mixed convection heat transfer of a high viscosity fluid contained in a two-dimensional rectangular tank subject to rolling motion. The study is motivated by the thermal design of heating systems of a tanker in a wavy sea. Basic equations are given for the body (tank)-fixed coordinate system considering inertia forces acting on the fluid in the tank including centrifugal force, Coriolis force, etc. The isotherms and flow velocity vectors are determined by the numerical solutions of the basic equations. Similarity parameters are introduced, and their influence on the heat transfer is systematically examined.     

FLOW BOILING OF WATER AND n -HEPTANE IN MICRO CHANNELS

Heat transfer characteristics during flow boiling in micro channels with rectangular cross section were studied using a thermographic measuring method. The characteristic length of the channels investigated was varied in a region of 300 w m up to 700 w m. The channels were designed as Joule heating pipes. Thus, the evaporation was achieved under conditions of nearly constant heat fluxes at the heating wall of the channel. The thermographic measuring method was used to examine the wall temperature. The high spatial and temporal resolution of this thermographic measuring method makes it possible to detect the axial position of the different boiling regions. Furthermore, it allows conclusions to be made on which flow conditions occur in the different sections of the channel. Experimental results are shown with water and n -heptane as the fluid to be vaporized. The results of measurements are discussed and a correlation is given of the location where the dryout starts with the characteristic parameters. The dependence of the pressure drop in such channels on the technological parameters has also been presented.     

Water management studies in PEM fuel cells, part IV: Effects of channel surface wettability, geometry and orientation on the two-phase flow in parallel gas channels

In this study, the effects of channel surface wettability, cross-sectional geometry and orientation on the two-phase flow in parallel gas channels of proton exchange membrane fuel cells (PEMFCs) are investigated. Ex situ experiments were conducted in flow channels with three different surface wettability (hydrophilically coated, uncoated, and hydrophobically coated), three cross-sectional geometries (rectangular, sinusoidal and trapezoidal), and two orientations (vertical and horizontal). Flow pattern map, individual channel flow variation due to maldistribution, pressure drop and flow visualization images were used to analyze the two-phase flow characteristics. It is found that hydrophilically coated gas channels are advantageous over uncoated or slightly hydrophobic channels regarding uniform water and gas flow distribution and favoring film flow, the most desirable two-phase flow pattern in PEMFC gas channels. Sinusoidal channels favor film flow and have lower pressure drop than rectangular and trapezoidal channels, while the rectangular and trapezoidal channels behave similarly to each other. Vertical channel orientation is advantageous over horizontal orientation because the latter is more prone to slug flow, nonuniform liquid water distribution and instable operation.     

A forced convection subcooled boiling model for nonuniform axial heat fluxes

The modeling of convective subcooled boiling of water flowing in round tubes subjected to nonuniform axial heat fluxes is described. The effects of different axial heat flux profiles are modeled using a local hypothesis; i.e. flow and thermal development are assumed to occur very rapidly in the subcooled boiling (SCB) flow regime. A computer code has been developed to predict the pressure drop, heat transfer coefficient and wall temperature for nonuniform axial heat fluxes, starting with a well-validated code for uniform axial heat fluxes. The predictions for some common nonuniform axial heat profiles are compared to the uniform heat flux case.     

System instability of evaporative micro-channels

In parallel evaporative micro-channels, system instability may occur in terms of cyclical fluctuations at a long period. This is due to the co-existence of the liquid phase flow at high mass flux and the two-phase flow at a lower mass flux among different parallel channels under the same total pressure drop. For a system at constant flow rate pumping, with a pressure regulating tank and a constant heating pre-heater, alternations between these two states of boiling and non-boiling could happen with a period of minutes. This cyclical system instability has been modeled, where the liquid phase flow occurs at conditions of high inlet subcooling and low surface heat flux that the boiling inception is hard to initiate. The system instability criteria are established in terms of a system binary states parameter, S, and a non-dimensional surface heat flux. This model has been validated experimentally.     

General characteristics of two-phase flow distribution in a compact heat exchanger

An experimental loop representing a compact plate heat exchanger was built up to study the two-phase distribution in the different header channels. The test section consists of a cylindrical horizontal header and eight rectangular channels in which the liquid and vapour flow rates are evaluated and the flow inside the header can be visualized. Several geometrical and functional parameters to study the two-phase distribution were tested using “HFE 7100” at a temperature close to 57 °C and a pressure close to 100 kPa. A flow pattern map in the header was built up using the different entry parameters on which a quantitative understanding of the two-phase distribution could be deduced.     

Fluid Flow Distribution and Heat Transfer in Plate-Fin Heat Exchangers

Single-phase and two-phase flow distribution in plate-fin heat exchangers and the influence of nonuniform fluid flow distribution on the thermal performance of such heat exchangers were experimentally investigated. The experimental results show that flow maldistribution can be a serious problem in plate-fin heat exchangers because of nonoptimized header configurations. The uneven distribution of two-phase flow in plate-fin heat exchangers is more pronounced than that of single-phase flow. It is shown that the uneven distributions result in a significant deterioration of the heat transfer performance. The relationship between the flow maldistribution characteristics and the resulting loss in heat exchanger effectiveness has been studied in this work. Certain improved header configurations with perforated plates were proposed in order to solve the maldistribution problem. It was found that the new header configurations could effectively improve the thermal performance of plate-fin heat exchangers. By changing the header configuration, the degree of flow and temperature nonuniformity in the plate-fin heat exchanger was reduced to 16.8% and 74.8%, respectively, under the main test condition.     

Steady-state performance of a two-phase natural circulation loop

Using both HEM and TEDFM the steady-state behaviour of a rectangular uniform cross section two-phase closed NCL has been studied through one-dimensional approach. Uniform heat flux for heating as well as cooling has been considered along with the constant physical properties, uniform pressure inside the loop and water-steam as the working fluid. For the given operating range and loop geometry, the circulation velocity lies in the turbulent regime. The flow rate monotonically increases with the loop height. However, with heat flux it initially increases, reaches maximum and then falls. This implies that the surface heat flux should match the loop geometry and other operating parameters to produce the maximum flow rate. It has also been observed that the cooling and heating lengths and the temperature of the liquid at the entry of the heating section have to be selected optimally to generate the maximum mass flow rate.     

Non-linear analyses of flow boiling in microchannels

Recently, four unstable boiling cases with different fluctuating amplitudes were observed in parallel silicon microchannels having a hydraulic diameter of 186 μm. These were: the liquid/two-phase alternating flow (LTAF) at two different heat fluxes, the continuous two-phase flow (CTF) at medium heat flux and medium mass flux, and the liquid/two-phase/vapor alternating flow (LTVAF) at high heat flux and low mass flux. In this paper, data of these unstable boiling cases are analyzed using the following methods: correlation coefficient, attractor reconstruction, correlation dimension and largest Lyapunov exponent. The processes responsible for appearance of chaotic oscillations in microchannels, such as nucleation, stability of bubbly flow, vapour core stability and vapour-phase flow stability, are discussed. It is shown that under certain conditions, the microchannels system works as a thermal oscillator. It was found that heat supplied to the microchannels increases the heating surface temperature while the appearance of the two-phase flow inside the channels decreases the heating surface temperature. The mechanism involving an increase in heating surface temperature is supported by phenomena of blocking the liquid flow in microchannels by the two-phase flow.     

Network Analysis of Nonuniformly Heated Evaporative Microchannel Systems

A network analysis was established to model the array of evaporative microchannels with possible nonuniformity heating as well as branching of the channels. Iterative solution of the evaporative microchannel network can be obtained using the Hardy–Cross method together with accurate two-phase head-loss correlations. Based on the experimental evidence, cross-cutting microchannels reduce the uneven flow distribution in parallel microchannels at nonuniform heating. Through this network analysis, it is also evident that cross-cutting grooves on parallel microchannels are effective in reducing nonuniform heating effects and enhancing the uniform wall temperature distribution.     

Experimental study of unsteady convective boiling in heated minichannels

Convective boiling in narrow channels may under specific conditions display an unsteady behavior. An experimental set-up has been elaborated to investigate heat and mass transfer and to analyze two-phase flow instabilities in rectangular microchannels with a hydraulic diameter of 889 μm. Depending on the operating conditions two types of behavior are observed: a steady state characterized by pressure drop fluctuations with low amplitudes (from 0.5 to 5 kPa/m) and no characteristic frequency; a non-stationary state of two-phase flow. The pressure signals exhibit fluctuations with high amplitudes (from 20 to 100 kPa/m) and frequencies ranging from 3.6 to 6.6 Hz. Steady and unsteady thermo-hydraulic behaviors depending on the two control parameters (heat flux and mass velocity) are analyzed and given in this paper.     

垂直并联多通道内高温高压汽-水两相流密度波型不稳定性的实验研究

在高压汽水两相流实验台上对垂直上升并联多通道中的汽 水两相流密度波型不稳定性进行了系统的试验研究,发现了并联多通道中汽液两相流密度波型不稳定性的主要特征,确定了系统压力、质量流速、入口过冷度、热负荷、进口及出口节流、可压缩容积等对该类不稳定性的影响;并将垂直并联多通道内高压汽液两相流的密度波型不稳定性与垂直并联双通道和单通道内的密度波不稳定进行了对比分析。所得结果可为大型直流锅炉和蒸汽发生器的设计提供依据。图13参10     

Hydrodynamics and heat transfer prior to onset of nucleate boiling in a rectangular microchannel heat sink

The conjugate heat transfer of flow boiling in a rectangular microchannel heat sink (MCHS) was modelled numerically to investigate the hydrodynamics and thermal responses of flow prior to the onset of nucleate boiling (ONB). Local hydrodynamics and thermal conditions leading to ONB are analysed numerically for different heat flux. The flow patterns of different modes of microconvection and mixed convective flows including the circulating flow, wavy flow and seeping flow were demonstrated and discussed. The numerical study proposes the mechanism leading to the first bubble nucleation which cover the initiation of fluid instability until the ONB. This work provides better understanding of the superheat induced flow instability and the progressive fluid convection under transient heating.     

Experiments on laminar flow and heat transfer in an elliptical duct

Experiments were performed to study the laminar developing and fully developed flow and heat transfer inside an elliptical duct having an aspect ratio of 0.5. The working fluid was air and two thermal situations were investigated, the first with the duct at a uniform temperature and the second when the wall temperature distribution is linear in the axial direction and does not vary transversely. The hydrodynamic results are presented in the form of a sequence of velocity profiles on the major and minor axes, measured at axial locations extending from the duct entrance to the fully developed regime. The axial drop in the static pressure due to the combined effect of the flow development and wall friction is also reported. The extended length necessary for static pressure development, expressed as x/Re D_h, was found to be 0.0345. The thermal information depicts the temperature development in the duct entrance by a series of temperature profiles on the major and minor axes. The thermal results encompass as well the Nusselt number and the thermal entrance length in each of the above two thermal situations. To the author's knowledge, theoretical solutions for the hydrodynamic flow development in the entrance of elliptical ducts do not exist. The present experimental fully developed dimensionless velocity and friction factor were compared to the analytic value of L. N. Toa [On some laminar-forced-convection problems, ASME J. Heat Trans. 83, 466–472 (1961)]. The percentage difference in the friction factor is 0.78%. The thermal development of the flow in the elliptical duct was studied analytically by N. T. Dunwoody [Thermal results for forced heat convection through elliptical ducts, J. appl. Meal. 29, 165–170 (1962)] and V. Javeri [Analysis of laminar thermal entrance region of elliptical and rectangular channels with the Kantorowich method, Wärme Stoffubert. 9, 85–98 (1976)] for the uniform and linear wall temperature ducts respectively. Both analyses assume either uniform or fully developed velocity profiles in the developing regime.     

Experimental and theoretical study on single-phase natural circulation flow and heat transfer under rolling motion condition

Experimental and theoretical studies of single-phase natural circulation flow and heat transfer under a rolling motion condition are performed. Experiments with and without rolling motions are conducted so the effects of rolling motion on natural circulation flow and heat transfer are obtained. The experimental results show the additional inertia caused by rolling motion easily causes the natural circulation flow to fluctuate. The average mass flow rate of natural circulation decreases with increases in rolling amplitude and frequency. Rolling motion enhances the heat transfer, and the heat transfer coefficient of natural circulation flow increases with the rolling amplitude and frequency. An empirical equation for the heat transfer coefficient under rolling motion is achieved, and a mathematical model is also developed to calculate the natural circulation flow under a rolling motion condition. The calculated results agree well with experimental data. Effects of the rolling motion on natural circulation flow are analyzed using the model. The increase in the flow resistance coefficient is the main reason why the natural circulation capacity decreases under a rolling motion condition.     

Mixed convection boundary layer flow over a thin vertical cylinder with localized injection/suction and cooling/heating

The effects of localized cooling/heating and injection/suction on the mixed convection flow on a thin vertical cylinder have been studied. The localized cooling/heating and (or) injection/suction introduce a finite discontinuity in the mathematical formulation of the problem which increases its complexity. In order to overcome this difficulty, a nonuniform distribution of wall temperature (heat flux) and surface mass transfer is considered at certain sections of the cylinder. The nonlinear coupled parabolic partial differential equations governing the mixed convection flow under boundary layer approximations have been solved numerically by using an implicit finite-difference scheme. The effects of the localized cooling/heating and (or) injection/suction on the heat transfer are found to be significant, but the effects of cooling/heating on the skin friction are comparatively small. The positive buoyancy force which assists the flow and the curvature parameter increase the skin friction and heat transfer.     

Liquid-solid separation phenomena of two-phase turbulent flow in curved pipes

The present study is to contribute some knowledge of phase separation phenomena of liquid-solid two-phase turbulent flow in curved pipes and provide a basis for the invention and development of a new type of curved pipe separator. Firstly, the solid-liquid two-phase flows in two-dimensional (2D) curved channels were numerically simulated using a two-way coupling Euler-Lagrangian scheme. Phase distribution characteristics of 2D curved channel two-phase flow were examined under conditions of different particle size, liquid flowrate and coil curvature. Based on the numerical results, the dynamic effects and contributions to phase separation of particle-subjected forces, including centrifugal force, drag force, pressure gradient force, gravity force, buoyancy force, virtual mass force and lift force, were exposed by kinematic and dynamic analysis along particle trajectories. Secondly, measurement of particle size and concentration profiles in helically coiled tube two-phase flow was conducted using a nonintrusive Malvern 2600 particle sizer based on laser diffraction. Particle size and concentration distribution characteristics of helically coiled tube two-phase flow and the effect of secondary flow on phase separation were analyzed based on experimental data.     

Unsteady nanofluid flow between two spinning expanding disks with continuous vertical motion under the influence of modified Hall effect

The principle aim of this article is to detect the effects of externally applied magnetic force in the nanofluid flow, flowing between two co-axially rotating and expanding disks where the upper disk is continuously moving vertically upward and downward. Also, the modified Hall Effect has been considered as an effective factor of the flow. The lower disk is vertically static. The rotation and vertical motion of the disks create a three-dimensional flow of nanofluid. Heat and mass density along with the motion of the flow has been analyzed under the variation in magnetic and Hall parameters. The findings have been compared with the results in Von Karman flow of nanofluid between two rotating and stretching disks. The velocity components have been largely influenced by magnetic and Hall parameters in case of downward movement of the upper disk. The fluid temperature is detected higher in case of upward velocity and lower in case of the downward motion of the upper disk. The heat transferability of the disks is effected differently at two different disks with the influence of magnetic force and Hall effect.     

Study on the mechanism of critical heat flux enhancement for subcooled flow boiling in a tube with internal twisted tape under nonuniform heating conditions

Critical heat flux (CHF) of subcooled flow boiling with water in a tube with an internal twisted tape under nonuniform heating conditions was experimentally investigated by direct current heating of a stainless steel tube. The boiling curve of the subcooled flow under a high heat flux was measured to confirm the characteristics of the nucleate boiling. The net vapor generation (NVG) point almost agreed with the Levy correlation. The increase of the CHF with an internal twisted tape under nonuniform heating conditions was explained by assuming an alternate development and disruption of the bubble boundary layer in which the bubble boundary layer is assumed to be disrupted when the heat flux is lower than the NVG heat flux. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25(5): 293–307, 1996     

Comparative Study of Thermal Performance of Parallel Plate and Rectangular Microchannel Counter Flow Heat Exchangers

This paper is aimed at comprehensive investigations of the thermal performance of parallel plate and rectangular microchannel counter flow heat exchangers based on axial conduction, number of transfer units, and non-dimensional power density. The geometrical parameters of the two configurations are optimized for a given heat transfer rate, effectiveness, and pressure drop. A reduced order model of rectangular micro channel counter flow heat exchanger is developed in which it is transformed into a hydrodynamically and thermally equivalent parallel plate micro heat exchanger. To improve the accuracy of the model, correction factors obtained from detailed computational fluid dynamics model are introduced. Various factors affecting the dimensionless power density of both the counter flow micro heat exchangers are studied. It is found that the axial conduction plays an important role on the performance of rectangular channel counter flow micro heat exchanger. In the limiting case where the channel aspect ratio tends to zero, the dimensionless power density of rectangular channel is found to approach that of a parallel plate counter flow micro heat exchanger.