Turbulent heat transfer of natural convection between two vertical parallel plates

We measure heat transfer coefficients of natural convection between two vertical smooth parallel plates heated uniformly in the laminar, transition, and turbulent regions. The heat transfer characteristics are experimentally investigated with changing width, δ, between the vertical parallel plates, wall heat flux, q_w, overall watercourse length, L,of the vertical parallel plate and heating conditions. For natural convection between the vertical parallel plates, in the turbulent region of , the heat transfer is strongly suppressed owing to the effect of combined convection. On the contrary, the heat transfer in the laminar region is enhanced due to the tunnel effect. These tendencies become pronounced with decreasing δ and increasing L.The location of the heat transfer reduction shifts downstream with increasing q_w under a fixed δ. Furthermore, under smaller δ, we cannot clearly distinguish the transition process in accordance with both the heat transfer enhancement in the laminar region and the heat transfer reduction in the turbulent region. © 2001 Scripta Technica, Heat Trans Asian Res, 31(1): 56–67, 2002     

Thermogravitational and thermocapillary convection heat transfer in concentric and eccentric horizontal,cylindrical annuli filled with two immiscible fluids

Laminar thermogravitational convection in concentric and eccentric horizontal, cylindrical annuli, filled with two immiscible fluids (water/air, water/silicon oil 10, water/silicon oil 100, Freon 113/ water) is studied numerically. Streamline and temperature distributions, local and average equivalent thermal conductivities are obtained over a wide range of Rayleigh number. The influence of thermocapillary convection (Marangoni convection) is similarly demonstrated for the water/air system in an annular enclosure.     

Turbulent convective transfer in plate heat exchangers

Readily available data on turbulent transfer in plate heat exchangers can be correlated by a heat transfer-energy dissipation analogy:

$\text{Nu}\text{g}{}_1\text{(pr, Vi)}\text{=C}{}_3\text{(fRe}{}_3\text{)}{}_{\mathrm{\delta }}$

in which the Nusselt number modified for changes in the Prandtl number and bulk to wall viscosity ratio Vi is related to the friction factor f and the Reynolds number. The exponent e is a weak function of the coefficient C₃ which depends on the corrugation geometry.When using chevron or herringbone type patterns the heat transfer depends significantly on the angle between the plate corrugation and the main flow direction. If this angle is π/4 the heat transfer per unit of mechanical energy dissipated is a maximum. Although maximum transfer (with maximum pressure drop) is obtained at π/2, a more practical angle giving high transfer at moderate pressure drops in 2π/5.     

Turbulent mass transfer in the mass transfer entry region for non-newtonian fluids

The effect of non-Newtonian flow behavior on turbulent mass transfer in the mass transfer entry region is investigated analytically. The predicted mass transfer rates in the mass transfer entry region are in good agreement with available experimental data and empirical correlations. The theoretical results for drag-reducing fluids show about 30 – 50% mass transfer rate reduction in the entry region which is less than that in the fully developed region.     

似Hele-Shaw盒中的直接接触式汽化传热及界面形态

应用似Hele—Shaw盒的窄缝实验设备。考察了向高黏热液中引入挥发性冷剂进行直接接触汽化传热的过程,获得了糖液的冷却曲线;分析了冷剂通入量和冷剂的入口温度对瞬问主体温度及容积传热速率的影响,并以“界面汽化热阱”效应概念对传热过程进行了分析。同时,研究了冷剂驱替高黏液体时的界面形态,对形成的黏性指进现象进行了图像分析并应用分形理论进行了阐述,获得了界面形态与两液体的黏性比的关系,并分析产生不同指征的原因及其对传热的影响。     

Peristaltic motion with heat and mass transfer of nano-Williamson fluids in two layers

In this paper, we have investigated the peristaltic motion with heat and mass transfer through a vertical channel divided into two equal regions, the right region filled with a clear non-Newtonian fluid obeying the Williamson model and the left region with a nano-Williamson fluid. The system is stressed by a gravity force with a uniform external magnetic field. The problem is modulated mathematically with a system of coupled nonlinear partial differential equations that describe the velocities, temperatures, and concentration of the fluids. The system of nondimensional, nonlinear, and partial differential equations is solved analytically with the homotopy perturbation method after using the approximations of low Reynolds number and long wavelength. The obtained solutions are functions of the physical parameters of the problem. Then, the effects of these parameters on velocities, temperatures, and concentration are discussed numerically and illustrated graphically through a set of figures. It is found that the parameters play an important role in controlling the solutions. It is shown that the stream function decreases on the left side and increases on the right side with an increase in the Wissenberger parameter and thermal conductivity ratio. Also, the temperature in the two regions increases with an increase in the thermophoretic parameter, whereas it decreases with an increase in the Brownian motion parameter. Furthermore, the concentration increases with an increase in the Brownian motion parameter and decreases with an increase in the thermophoretic parameter.     

Forced convection heat transfer between permeable rotating slender cylinders and power-law fluids

A new analysis is presented for forced convection of power-law fluids past rotating slender cylinders with fluid suction/injection at the wall. After a suitable coordinate transformation to reduce the complexity of the governing boundary-layer equations, the resulting nonlinear coupled differential equations were solved with an implicit finite difference scheme. Of interest were the effects of transverse curvature, power-law viscosity index, the type of thermal boundary condition and the generalized Prandtl number on the local skin friction coefficient and Nusselt number of rotating cylinders with porous surfaces. In general, body spin enhances convection heat transfer and increases the skin friction coefficient. Fluid injection into the boundary layer reduces and suction increases the local skin friction group. Higher Prandtl numbers increase the local heat transfer group especially in the presence of fluid withdrawal. Dilatant fluids exhibit a distinctively different behavior with respect to the heat transfer group when compared to pseudoplastics. The two thermal boundary conditions yield quite similar results.     

Hyperbolic heat transfer in generalized Newtonian fluids

The present paper is concerned with the modeling of hyperbolic heat transfer in generalized Newtonian fluids. A general procedure, developed within the framework of thermodynamics of irreversible processes, is proposed to obtain constitutive relations that verify automatically the second law of thermodynamics and the principle of material objectivity. Such a thermodynamic approach allows a rational identification of the terms responsible for the thermomechanical coupling in the heat equation, which is a first step to better understand its influence on the fluid behavior.     

A simplified model of direct-contact heat transfer in desalination system utilizing LNG cold energy

With the increasingly extensive utilization of liquefied natural gas (LNG) in China today, sustainable and effective using of LNG cold energy is becoming increasingly important. In this paper, the utilization of LNG cold energy in seawater desalination system is proposed and analyzed. In this system, the cold energy of the LNG is first transferred to a kind of refrigerant, i.e., butane, which is immiscible with water. The cold refrigerant is then directly injected into the seawater. As a result, the refrigerant droplet is continuously heated and vaporized, and in consequence some of the seawater is simultaneously frozen. The formed ice crystal contains much less salt than that in the original seawater. A simplified model of the direct-contact heat transfer in this desalination system is proposed and theoretical analyses are conducted, taking into account both energy balance and population balance. The number density distribution of two-phase bubbles, the heat transfer between the two immiscible fluids, and the temperature variation are then deduced. The influences of initial size of dispersed phase droplets, the initial temperature of continuous phase, and the volumetric heat transfer coefficient are also clarified. The calculated results are in reasonable agreement with the available experimental data of the R114/water system.     

Free-convective heat transfer in fluids with non-uniform volumetric heat generation

The characteristics of natural convection of a heat-generating fluid with non-uniform distribution of volumetric heat release have been studied theoretically. The analysis was based on analytical estimates method and numerical simulation. It has been found that under particular conditions the details of volumetric heat release distribution over horizontal cross-sections of the liquid pool do not affect the convectional heat transfer characteristics. The vertical distribution of the horizontal cross-sectional mean value of volumetric heat generation completely determines the distribution of temperature in the bulk as well as the heat flux to the cooled boundary.     

Turbulent heat transfer in a horizontal helically coiled tube

An experiment has been conducted in detail to study the turbulent heat transfer in horizontal helically coiled tubes over a wide range of experimental parameters. We found that the enhancement of heat transfer in the coils results from the effects of turbulent and secondary flows. With Reynolds number increasing to a high level, the contribution of the secondary flow becomes less to enhance heat transfer, and the average heat transfer coefficient of the coil is closer to that in straight tubes under the same conditions. The local heat transfer coefficients are not evenly distributed along both the tube axis and the periphery on the cross section. The local heat transfer coefficients on the outside are three or four times those on the inside, which is half of the average heat transfer. A correlation is proposed to describe the distribution of the heat transfer coefficients at a cross section. The average cross-section heat transfer coefficients are distributed along the tube axis. The average value at the outlet section should not be taken as the average heat transfer coefficient. © 1999 Scripta Technica, Heat Trans Asian Res, 28(5): 395–403, 1999     

Turbulent heat transfer enhancement in a heat exchanger using helically corrugated tube

The augmentation of convective heat transfer in a single-phase turbulent flow by using helically corrugated tubes has been experimentally investigated. Effects of pitch-to-diameter ratio (P/D_H = 0.18, 0.22 and 0.27) and rib-height to diameter ratio (e/D_H = 0.02, 0.04 and 0.06) of helically corrugated tubes on the heat transfer enhancement, isothermal friction and thermal performance factor in a concentric tube heat exchanger are examined. The experiments were conducted over a wide range of turbulent fluid flow of Reynolds number from 5500 to 60,000 by employing water as the test fluid. Experimental results show that the heat transfer and thermal performance of the corrugated tube are considerably increased compared to those of the smooth tube. The mean increase in heat transfer rate is between 123% and 232% at the test range, depending on the rib height/pitch ratios and Reynolds number while the maximum thermal performance is found to be about 2.3 for using the corrugated tube with P/D_H = 0.27 and e/D_H = 0.06 at low Reynolds number. Also, the pressure loss result reveals that the average friction factor of the corrugated tube is in a range between 1.46 and 1.93 times over the smooth tube. In addition, correlations of the Nusselt number, friction factor and thermal performance factor in terms of pitch ratio (P/D_H), rib-height ratio (e/D_H), Reynolds number (Re), and Prandtl number (Pr) for the corrugated tube are determined, based on the curve fitting of the experimental data.     

Modelling of heat transfer between two rollers in dry friction

An analysis of heat transfer between two rollers in dry friction is presented in this paper. The contact is peripheral and is assumed to be imperfect. The heat transfer at the interface is modelled by a thermal contact resistance. The heat flux is generated by dry friction at the interface. The two rollers are cooled by convection. A numerical model has been developed to determine the steady state temperature in rollers. Taking into account the transport phenomenon due to motion, the mesh is correlated with the velocity. The accuracy of the mesh is validated by comparison with an available analytical solution developed for a single roller in rotation. The thermal behaviour is analysed with respect to: (i) the velocity, (ii) the heat convection coefficient, and (iii) the thermal contact resistance. The evolutions of the temperature and the partition coefficient of frictional heat are presented and discussed.     

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.     

Heat degradation studies of solar heat transfer fluids

Destructive distillation of solar heat transfer fluids was conducted to determine the types of pyrolytic products which might be formed in solar collectors under conditions of stagnation or malfunction. The distillates were analyzed by gas chromatography/mass spectrometry (GC/MS) to determine the types of compounds which were present and the minor components which might be formed. Dehydration products were formed from ethylene and propylene glycols with ethylene oxide and propylene oxide being minor products. A high aromatic petroleum heat transfer fluid yielded distillation fractions enriched in quinolines or isoquinolines and methylated derivatives thereof. Fractionation of this aromatic heat transfer fluid showed that the basic fraction of unheated fluid also contained aza-arenes which exhibited mutagenic activity in the Ames' bioassay. GC/MS showed that the basic fraction from unheated heat transfer fluid also contained benzoquinolines which appeared to be lacking in heat degraded samples of the same fluid. Fractionation of heat transfer fluids, in particular petroleum-based fluids, may often be necessary in order to concentrate minor components so that they can be identified by GC/MS and in order to detect mutagenic activities without interference from cytotoxic components.     

A new heat transfer correlation for supercritical fluids

A new method of heat transfer prediction in supercritical fluids is presented. Emphasis is put on the simplicity of the correlation structure and its explicit coupling with physical phenomena. Assessment of qualitative behaviour of heat transfer is conducted based on existing test data and experience gathered from open literature. Based on phenomenological analysis and test data evaluation, a single dimensionless number, the acceleration number, is introduced to correct the deviation of heat transfer from its conventional behaviour, which is predicted by the Dittus-Boelter equation. The new correlation structure excludes direct dependence of heat transfer coefficient on wall surface temperature and eliminates possible numerical convergence. The uncertainty analysis of test data provides information about the sources and the levels of uncertainties of various parameters and is highly required for the selection of both the dimensionless parameters implemented into the heat transfer correlation and the test data for the development and validation of new correlations. Comparison of various heat transfer correlations with the selected test data shows that the new correlation agrees better with the test data than other correlations selected from the open literature.     

Turbulent impinging jet heat transfer enhancement due to intermittent pulsation

A numerical study was carried out of heat transfer under a pulsating turbulent slot impinging jet. The jet velocity was varied in an intermittent (on–off) fashion. The effects of the time-mean jet Reynolds number, temperature difference between the jet flow and the impinging surface, nozzle-to-target distance as well as the frequency on heat and mass transfer were examined. The numerical results indicate significant heat transfer enhancement due to intermittent pulsation of the jet flow over a wide range of conditions for both cooling and heating cases. Simulations of the flow and temperature fields show that the instantaneous heat transfer rate on the target surface is highly dependent on the hydrodynamic and thermal boundary layer development with time.     

Turbulent heat transfer on the surface of a steam-chugging bubble

The length and velocity scales of the turbulent eddies at the surface of a type of steam-chugging bubble (the detached bubble) are derived from a physical model. The corresponding turbulent heat transfer result, which contains no free parameters, is $\text{Nu}\text{= 0.04}\text{Re}{}^{\mathrm{<mtext>7</mtext><mtext>8</mtext>}}\text{Pr}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$. A comparison with the available experimental data shows excellent agreement.