Entropy generation in a pipe due to non-Newtonian fluid flow: Constant viscosity case

Non-Newtonian fluid flow in a pipe system is considered and a third grade non-Newtonian fluid is employed in the analysis. The velocity and temperature distributions across the pipe are presented. Entropy generation number due to heat transfer and fluid friction is formulated. The influences of non-Newtonian parameter and Brinkman number on entropy generation number are examined. It is found that increasing the non-Newtonian parameter reduces the fluid friction in the region close to the pipe wall. This in turn results in low entropy generation with increasing non-Newtonian parameter. Increasing Brinkman number enhances the fluid friction and heat transfer rates; in which case, entropy number increases with increasing Brinkman number.     

Investigation of turbulent heat transfer and nanofluid flow in a double pipe heat exchanger

In present study, heat transfer and turbulent flow of water/alumina nanofluid in a parallel as well as counter flow double pipe heat exchanger have been investigated. The governing equations have been solved using an in-house FORTRAN code, based on finite volume method. Single-phase and standard k-ε models have been used for nanofluid and turbulent modeling, respectively. The internal fluid has been considered as hot fluid (nanofluid) and the external fluid, cold fluid (base fluid). The effects of nanoparticles volume fraction, flow direction and Reynolds number on base fluid, nanofluid and wall temperatures, thermal efficiency, Nusselt number and convection heat transfer coefficient have been studied. The results indicated that increasing the nanoparticles volume fraction or Reynolds number causes enhancement of Nusselt number and convection heat transfer coefficient. Maximum rate of average Nusselt number and thermal efficiency enhancement are 32.7% and 30%, respectively. Also, by nanoparticles volume fraction increment, the outlet temperature of fluid and wall temperature increase. Study the minimum temperature in the solid wall of heat exchangers, it can be observed that the minimum temperature in counter flow has significantly reduced, compared to parallel flow. However, by increasing Reynolds number, the slope of thermal efficiency enhancement of heat exchanger gradually tends to a constant amount. This behavior is more obvious in parallel flow heat exchangers. Therefore, using of counter flow heat exchangers is recommended in higher Reynolds numbers.     

A survey of correlations for heat transfer and pressure drop for evaporation and condensation in plate heat exchangers

Plate Heat Exchangers (PHEs) are used in a wide variety of applications including heating, ventilation, air-conditioning, and refrigeration. PHEs are characterized by compactness, flexible thermal sizing, close approach temperature, and enhanced heat transfer performance. Due to their desirable characteristics, they are increasingly utilized in two-phase flow applications. Detailed research on heat transfer and fluid flow characteristics in these types of exchangers is required to design and use plate heat exchangers in an optimal manner. This paper reviews the available literature on the correlations for heat transfer and pressure drop calculations for two-phase flow in PHEs as an initial process step in order to understand the current research status. Comparative evaluations for some of the existing correlations are presented in the light of their applicability to different refrigerants. Overall, there is a significant gap in the literature regarding two-phase heat transfer and fluid flow characteristics of these types of exchangers.     

Heat transfer in turbulent nanofluids: Separation flow studies and development of novel correlations

Convective heat transfer plays a significant role in numerous industrial cooling and heating applications. This method of heat transfer can be passively improved by reconfiguring flow passage, fluid thermophysical properties, or boundary conditions. The broader scope of nanotechnology introduced several studies of thermal engineering and heat transfer. Nano-fluids are one of such technology which can be thought of engineered colloidal fluids with nano-sized particles. In the present study, turbulent forced convection heat transfer to nanofluids in an axisymmetric abrupt expansion heat exchanger was investigated experimentally. During heat transfer investigation, the functionalized multiwalled carbon nanotubes (MWCNT-COOH), polycarboxylate functionalized graphene nanoplatelets (F-GNP), SiO₂ and ZnO water-based nanofluids were used. The convective heat transfer coefficient of fully developed turbulent flow of nanofluids flowing through an abrupt enlargement with the expansion ratio (ER) of 2 was experimentally determined at a constant wall heat flux of 12,128.56 W/m². The experiments were conducted at the Re ranges of 4000–16,000. The observed Nusselt numbers were higher than in the case of fully developed pipe flow indicating the level of the turbulent transport is high even though the recirculating velocities were a few percentages of the bulk mean velocity. The effect of Reynolds number and nanofluid’s volume concentration on heat transfer and friction losses were studied, where all the results reveal that with the increase of weight concentration and Reynolds number, the local Nusselt number enhanced at the increment of axial ratios in all the cases showing greater heat transfer rates than those of the base fluids. Comparison between the examined four types of nanofluids, show that the carbon-based nanofluids have a greater effect on enhancing heat transfer (33.7% and 16.7% heat transfer performance improvement for F-GNP and MWCNT nanofluids respectively at 0.1 wt% concentration) at the downstream of the sudden expansion pipe. There is no reported work dealing with the prediction of the local Nusselt number at the distance equivalent to the axial ratio and flow through sudden expansion. So far, two excellent correlations for the Local Nusselt number are proposed with reasonably good accuracy. Furthermore, a new correlation is developed for the average Nusselt number.     

Effects of tangential and radial velocity on fluid flow and heat transfer for flow through a pipe with twisted tape insert—laminar flow

The present study reports the numerical analysis of fluid flow and heat transfer in a pipe with full length twisted tape insert. The investigation is carried out for five different twist ratios of 4, 5, 6, 8 and 10 at 100 ≤ Re ≤ 1000. The velocity field in terms of streamwise, tangential and radial velocity and temperature field are studied as a function of Reynolds number and twist ratio. The variation of friction factor and Nusselt number with Reynolds number for different twist ratios is also presented. The heat transfer enhancement due to insertion of twisted tape mainly comes from the tangential and radial components of velocities, which are regarded as secondary fluid motion. It is evident from the results that with increase in Reynolds number the axial convection increases. However, with the decrease in the twist ratio, the tangential and radial convection increases, leading to increased heat transfer. The secondary flow affects the thermal boundary layer inside the tube and increases the cross-flow mixing, which increases the heat transfer. The correlations for prediction of friction factor and Nusselt number based on the numerical data are also proposed.     

An experimental study on flow patterns and heat transfer characteristics during cryogenic chilldown in a vertical pipe

In the present paper, the experimental results of a cryogenic chilldown process are reported. The physical phenomena involve unsteady two-phase vapor–liquid flow and intense boiling heat transfer of the cryogenic fluid that is coupled with the transient heat conduction inside pipe walls. The objective for the present study is to compare the chilldown rates and flow patterns between the upward flow and downward flow in a vertical pipe. Liquid nitrogen is employed as the working fluid and the test section is a vertical straight segment of a Pyrex glass pipe with an inner diameter of 8 mm. The effects of mass flow rate on the flow patterns, heat transfer characteristics and interface movement were determined through experiments performed under several different mass flow rates. Through flow visualization, measurement and analysis on the flow patterns and temperature variations, a physical explanation of the vertical chilldown is given. By observing the process and analyzing the results, it is concluded that pipe chilldown in a vertical flow is similar to that in microgravity to some extent.     

Application of CNT nanofluids in a turbulent flow heat exchanger

Nanofluids have received much attention since its discovery owing to its enhanced thermal conductivity and heat transfer characteristics which makes them a promising coolant in heat transfer application. In this study, the enhancement in heat transfer of carbon nanotube (CNT) nanofluids under turbulent flow conditions is investigated experimentally. The CNT concentration was varied from 0.051 to 0.085 wt%, respectively. The nanofluid suspension was stabilised by gum arabic through a process of homogenisation and water bath sonication at 25 °C. The flow rate of cold fluid (water) is varied from 1.7 to 3 L/min, while flow rate of the hot fluid is varied between 2 and 3.5 L/min. Thermal conductivity, density, and viscosity of the nanofluids are also measured as a function of temperature and CNT concentration. The experimental results were validated with theoretical correlations for turbulent flow available in the literature. Results showed an enhancement in heat transfer between 9% and 67% as a function of temperature and CNT concentration.     

板式单环路脉动热管的流动和传热特性

以制冷剂R245fa为工质对常规以及带连通通道的板式单环路脉动热管流动特性进行了可视化研究,并对传热性能进行了测试,考察了充液率和加热功率的影响规律。结果表明:对于常规环路,充液率一定时,随加热功率增加,工作方式依次经历振荡流、有方向改变的循环流、定向循环流,且充液率越高,出现定向循环的加热功率越低;连通通道提供了流动的额外通路,工作方式更具振荡特性,定向循环在高充液率、高功率下才能出现。在合适的充液率下,连通通道不但能降低热阻,而且能提高传热极限。     

Analysis of the heat transfer and friction factor correlations influence in the prediction of evaporating flows inside tubes

A methodology for analysing the influence of the heat transfer and friction factor correlations in the prediction of the two-phase flows inside horizontal ducts under evaporation phenomena is presented. An experimental unit based on single stage vapor compression refrigerating system with two parallel evaporation devices has been built to work under real refrigeration conditions. The first evaporation device consists of a double pipe evaporator which allows determining the heat flux through the pipe. The second device is an electrically heated pipe evaporator with uniformly distributed temperature and pressure sensors along the fluid path. The experimental data of temperature and pressure distribution along the smooth heated duct is compared with a selected set of heat transfer and friction factor correlations through a detailed numerical evaporation model. The aim of this paper is to determine possible criteria to select the most suitable heat transfer and friction factor correlations available.     

Non-Newtonian fluid flow in annular pipes and entropy generation: Temperature-dependent viscosity

Non-Newtonian fluid flow in annular pipes is considered and the entropy generation due to fluid friction and heat transfer in them is formulated. A third-grade fluid is employed to account for the non-Newtonian effect, while the Reynolds model is accommodated for temperature-dependent viscosity. Closed-form solutions for velocity, temperature, and entropy fields are presented. It is found that entropy generation number increases with reducing non-Newtonian parameter, while it is the reverse for the viscosity parameter, which is more pronounced in the region close to the annular pipe inner wall.     

Effects of uniform radial electric field on the MHD heat and fluid flow due to a rotating disk

The flow and heat transfer of an incompressible electrically conducting fluid over a rotating infinite disk are studied in the present paper. The disk finds itself subjected to a uniform normal magnetic field. The particular interest lies in searching for the effects of an imposed radial electric field on the behavior of the physical flow. The gradient of an electric potential generated on the disk penetrates through the fluid and greatly influences the boundary layer formation. The presented model representing the fluid motion is a general case since it reduces to the traditional Karman’s viscous pump when the electric potential is ignored. The governing Navier–Stokes and Maxwell equations of the constructed model together with the energy equation are converted into self-similar forms using suitable similarity transformations. The flow and thermal boundary layers are shown to be much affected by the presence of a uniform radial electric parameter. Some parameters of fundamental physical significance such as the surface shear stresses in the radial and tangential directions and the heat transfer rate are numerically evaluated. The effects of electric conductivity of the disk on the flow and forced convection heat transfer are further discussed.     

Heat Transfer in Meshed Metallic Materials with Interchannel Transpiration and Two-Dimensional Intermesh Flow of a Heat-Transfer Fluid

Some experimental data on the heat transfer in different meshed metallic materials with interchannel heat-transfer fluid transpiration and two-dimensional intermesh flow are presented. It is established that the thermal conductivity of a wire mesh material and the relative pathway, intermesh velocity, and thermophysical properties of a heat-transfer fluid has an effect on the heat transfer in meshed metallic materials with the two-dimensional flow of a single-phase heat-transfer fluid.     

Heat transfer under conditions of turbulent pipe flow of electrically conducting liquid in longitudinal magnetic field in the region of hydrodynamic stabilization

Numerical simulation is performed of heat transfer under conditions of turbulent pipe flow in the vicinity of entry to longitudinal magnetic field. Use is made of the model of turbulence which was previously employed for performing calculations in the region of stabilized flow and heat transfer. The model describes the suppression of turbulence by the magnetic field and the laminarization of turbulent (at the pipe inlet) flow. The calculation results agree well with the experimental data on heat transfer and temperature profiles in the initial thermal region. The effect made on heat transfer by Joule heat release from electric currents caused by turbulent fluctuations is investigated.     

Fluid flow and heat transfer characteristics of separation and reattachment flow over a backward-facing step

In this study, a direct numerical simulation of the fluid flow and heat transfer characteristics of separation and reattachment flow at a backward-facing step is presented. A computer program of FORTRAN code is used to solve the governing equations according to finite volume method. The effects of the Reynolds number and expansion ratio on the fluid flow and heat transfer characteristics are investigated. The size of the primary recirculation zone increases with the reduction of expansion ratio and the fluctuation of isotherms increased with the increase of Reynolds number. The periodic characteristics and the dissimilarity between Nu and Cf appear in the transitional flow regime. The rotating fluids in the reattachment region increase the flow instability and the interchange of the hot and cold fluids increases heat transfer instability. The combined effects of flow instability and heat transfer instability play an important role in the formation of the dissimilarity between Nu and Cf.     

Thermodynamics of active magnetic regenerators: Part I

Cycle-averaged relationships for heat transfer, magnetic work, and temperature distribution are derived for an active magnetic regenerator cycle. A step-wise cycle is defined and a single equation describing the temperature as a function of time and position is derived. The main assumption is that the convective interaction between fluid and solid is large so that thermal equilibrium between fluid and solid exists during a fluid flow phase (regeneration). Relations for the temperatures at each step in the cycle are developed assuming small regenerative perturbations and used to derive the net cooling power and magnetic work at any location in the AMR. The overall energy balance expression is presented with transformations needed to relate the boundary conditions to effective operating temperatures. An expression is derived in terms of operating parameters and material properties when each location is regeneratively balanced; this relation indicates needed conditions so the local energy balance will satisfy the assumed cycle. By solving the energy balance expression to determine temperature distribution one can calculate work, heat transfer, and COP.     

流体横掠泡沫金属中等温光管的流动与传热分析

基于Brinkman-Forchheimer-extended Darcy流动模型和局部热平衡传热理论建立了流体横掠泡沫金属中等温光管的对流传热控制方程组,运用Runge-Kutta法和"打靶法"对方程组进行了数值求解,依据数值计算结果对流体流动与传热性能进行了分析,并得出了对流传热的Nusselt数关联式。结果表明,泡沫金属的孔隙率和孔密度(ppi)对强化传热起着至关重要的作用,但它的存在同时也增大了压力降,这为泡沫金属在换热器等化工设备上的实际应用提供了理论依据。     

CtFD-based correlations for the thermal–hydraulics of an HTS current lead meander-flow heat exchanger in turbulent flow

The Karlsruhe Institute of Technology is responsible for the design, construction and testing of the high temperature superconductor (HTS) current leads for the Wendelstein 7-X stellarator and for the JT-60SA tokamak. These HTS current leads mount a heat exchanger of the meander-flow type, in which the helium flows between the fins and is forced to cross flow with respect to the central Cu bar, which actually carries the current. Since an important issue in the operation of the HTS current lead is the optimization of the cooling power consumption, the helium thermal–hydraulics in such complex geometry becomes rather important.In this paper we extend a computational thermal fluid dynamics (CtFD) technique, previously introduced by the same authors and validated on short samples of meander flow heat exchanger and on the W7-X HTS current lead prototype, to a systematic analysis of the helium thermal-fluid dynamics inside different meander flow geometries. The first aim is to clarify under what operative conditions the flow regime can be considered turbulent and how the pressure drop as well as the heat transfer are related to the geometrical parameters and to the flow conditions. From the results of this analysis, correlations for the pressure drop and the heat transfer in the meander flow geometry have also been derived, which are applicable with good accuracy for the design of HXs over a broad range of geometries.     

Mixed convective flow and heat transfer in a vertical helicoidal pipe with finite pitch

Fully developed laminar mixed convective flow and heat transfer in a vertical helicoidal pipe with finite pitch is studied numerically in this paper. The centrifugal, buoyancy, and torsion forces created by the pitch of the helix are considered. The thermal boundary conditions are uniform heat flux axially and uniform wall temperature peripherally. The velocity, the temperature profiles, the friction factor, and the Nusselt number are obtained. The results indicate that torsion can significantly change the axial and secondary flow patterns and temperature distributions in the helicoidal pipe with a fixed pressure gradient. Torsion can also remarkably affect the flow rate, the friction factor, and the Nusselt number. However, at a fixed Dean number, there is only slight difference in the Nusselt numbers of the upward and downward flow.The results presented in this paper were obtained in the course of research sponsored by the National Science Foundation (NSF) under Grant No. CTS-9017732     

Entwicklung und experimentelle Untersuchung neuer Regeneratorkonzepte für regenerative Gaskreisprozesse am Beispiel einer Vuilleumier-Wärmepumpe

The Stirling cycle and the Vuilleumier-cycle are well-known representatives of regenerative gas cycles. Regenerators are a fundamental component of these cycles. The task of a regenerator is to take up heat from the working fluid passing through the regenerator matrix and to restore this heat to the working fluid on its way back. These tasks define the requirements that have to be met by the regenerator material. The heat transfer and the pressure drop characteristics of the regenerator matrix must be in an acceptable range. Additionally, with regard to series production, the confectioning of the matrix material and the cost of production and after costs are of great interest. Up to now, as a rule, wire screens and wire gauzes have been used in regenerative gas cycle machines. Indeed, these solutions are suitable from a thermodynamic point of view, but they suffer from the disadvantage of high manufacturing costs. For that reason new regenerator concepts have been developed in the present work. The applicability of new materials as well as of new configurations of common materials in regenerative gas cycle machines has been investigated. Finally, all these regenerator concepts have been evaluated with regard to manufacturing, engineering and economic aspects.     

Entwicklung und experimentelle Untersuchung neuer Regeneratorkonzepte für regenerative Gaskreisprozesse am Beispiel einer Vuilleumier-Wärmepumpe

The Stirling cycle and the Vuilleumier-cycle are well-known representatives of regenerative gas cycles. Regenerators are a fundamental component of these cycles. The task of a regenerator is to take up heat from the working fluid passing through the regenerator matrix and to restore this heat to the working fluid on its way back. These tasks define the requirements that have to be met by the regenerator material. The heat transfer and the pressure drop characteristics of the regenerator matrix must be in an acceptable range. Additionally, with regard to series production, the confectioning of the matrix material and the cost of production and after costs are of great interest. Up to now, as a rule, wire screens and wire gauzes have been used in regenerative gas cycle machines. Indeed, these solutions are suitable from a thermodynamic point of view, but they suffer from the disadvantage of high manufacturing costs. For that reason new regenerator concepts have been developed in the present work. The applicability of new materials as well as of new configurations of common materials in regenerative gas cycle machines has been investigated. Finally, all these regenerator concepts have been evaluated with regard to manufacturing, engineering and economic aspects.