Heat Transfer Aspects Research Articles

Heat and Mass Transfer and Energy Aspects in Combined Infrared-Convective Drying of Bee-Pollen

A hybrid drying process based on the application of infrared (IR) radiation combined to non-heated air flow was proposed for bee-pollen drying. The IR source was located at a distance of 32 cm from the samples. Experiments in the hybrid dryer were conducted with pollen grains arranged in a single layer and exposed to three radiation intensity levels (150, 300 and 450 W/m2) and three air velocity levels (1.0, 2.0 and 3.0 m/s) at 25°C. The surface temperature of the material was measured during drying through an optical pyrometer. The effects of the process parameters on the drying kinetics and specific energy consumption were evaluated. Experimental results showed that for any particular air velocity increasing IR heat flux led to an intensification of mass transfer rate, decreasing drying time and thus reducing energy consumption. Minimum energy consumption in hybrid frying of pollen was associated with the application of the highest radiation intensity and lowest air velocity. It was also found that cooling effect increased with air velocity, which resulted in a decreased driving force for mass transfer and increased drying time and energy consumption.

Surface Scraped Heat Exchanger for cooling Newtonian fluids and enhancing its heat transfer characteristics, a review and a numerical approach

This study is an attempt to identify the factors influencing heat transfer in a Surface Scraped Heat Exchanger (SSHE) and determining its significance in order to facilitate the use of SSHE in various applications. These types of exchangers are employed to heat/cool fluids with high viscosity, sensitive to temperature variations and high affinity for deposition. The rate of the fluid entering the exchanger, revolution velocity of the rotor, shape and the number of blades, and mechanism of applying flux (fixed or variable) to the exchanger are the factors their heat transfer aspects investigated. After a comprehensive review of previous works, as a noble mode of elaborating SSHE, the exchanger is studied numerically to reveal its performance in cooling mode. It is found that, amongst the parameters studied, the revolution velocity of rotor is the most effective one and increasing the RPM improves the SSHE heat transfer performance significantly.

Experimental investigation on an axisymmetric turbulent jet impinging on a concave surface

An experimental study of an axisymmetric turbulent jet impinging on a semi-cylindrical concave surface is carried out using Particle Image Velocimetry (PIV). The measurements are performed on two orthogonal planes: parallel and perpendicular to the axis of the cylinder. The study is focused on the impinging jet, the generated wall jet and on the boundary layer developing on the cylinder surface. Linear stochastic estimation is applied for identifying the dominant structures in the flow and coupling then with other relevant parameters as the peak r.m.s. velocity of the wall jet to estimate secondary peak in Nusselt number distribution along the wall. Measurements are performed for two different surface curvatures and four nozzle to target distances (L/d ratio) at a Reynolds number equal to Re≈30,000. The mean and r.m.s. velocity profiles are investigated and the evolution of half-velocity width close to the wall is illustrated for both planes of interest. The results show a strong lateral movement of the flow brought about by the surface geometry for curved surface, especially for large L/d ratios. The r.m.s. profiles indicate higher values along the curved surface with respect to the quasi-flat one. The results show the most probable position of secondary peak in the Nusselt number distribution along quasi-flat and curved surfaces to be located at y/d=3.2 and 4.39<s/d<5, respectively. These results can be used toward interpretation of heat and mass transfer aspects on concave surfaces, and are also useful for benchmarking future numerical studies.

Optimal Design for Compact Heat Exchanger (Che) by Heat Transfer Viewpoint as an Air Pre-Heater

AbstractIn the provided research, the design of CHE (Compact Heat Exchanger) is evaluated and discussed from the heat transfer aspect. Benefiting from present equations and considering the objective concepts, the procedural chart is proposed for achieving optimal design. The main goal of this research study is implementing a new algorithm for optimization to modify a conventional design of CHE. Nonlinear gradient mathematical modeling with different scenarios on free or related variables is developed to cover the purpose of maximizing total heat transfer capacity. By mathematical programming analysis, a model has been provided for optimal design and developed in the GAMS (Generalized Algebraic Modelling System) software. Also for further model test rig development purpose, the proposed model has been incorporated in Matlab software using independent variants and the accuracy of the responses was again evaluated. The comparison indicated 109W/K difference in the exchanged thermal energy rate compared to the optimal exchanger operation conditions. After introducing case study to this model, an acceptable response with 0.997W/K difference on optimal point was achieved. Solving the model indicated 0.833W/K difference with the optimal point, which confirms the resulted technical responses.

Developing a Remote Laboratory for Heat Transfer Studies

Remote labs that enable students to conduct real-world experiments at a distance using a computer and Web-based tools can be the only realistic method of performing practical experiments in the context of distance learning.&#x0D; eSience Tempus project "Maghreb Network of remote labs" offers an innovative pedagogical approach which integrates eLearning and is complementary to classroom learning. The main objective of this project is to create an efficient remote labs network in the Maghreb region for the modernization of higher education in technological sciences. &#x0D; The current paper summarizes the work carried out on a heat exchanger bench to be fully accessed and controlled remotely. Through this new device a student of Mechanical Engineering is introduced to many fundamental aspects of heat transfer.

Modeling of Transport Phenomena in Low-Head Direct-Chill Caster for AA7050 Alloy

As an extension of earlier work, a three-dimensional numerical model has been developed for an industrial-scale low-head direct-chill slab casting process for the long freezing range aluminum alloy AA7050. The model has taken into account the coupled nature of the turbulent melt flow and the solidification heat transfer aspect of the direct-chill casting process. Computer simulations were performed to predict the velocity and temperature fields, the sump profile, the mushy thickness, and the shell thickness at the exit of the mold. Specifically, the aforementioned results were obtained for four casting speeds, varying from 60 to , for three metal–mold effective heat transfer boundary conditions, varying from 1.0 to , and for three pouring temperatures of inlet melt, namely, 645, 661, and 693°C. A stepwise change of the cooling water temperature in the mold and impingement and free-streaming regions, were considered to reflect the temperature history of the cooling water conditions in the industry. The importance of the present study lies in the fact that, by rolling the cast slabs from the direct-chill process, the obtained plates, sheets, strips, and foils of the aforementioned alloy are used extensively by the aerospace industry.

Local Temperature Evolution during Nanoparticle Hyperthermia Probed by Fluorescence Thermometry

Here we investigate the spatial and temporal evolution of temperature around heating nanoparticles held in varies spatial geometries. We compare isolated nanoparticles to ones aligned along an axis inside magnetotactic bacteria, to cell-bound ones, whereby the density is varied to change the particle-particle interaction. The nanoparticles or the cell membrane are labeled with fluorophores whose fluorescence intensity and lifetime are reduced with increasing temperature (TAMRA or Dylight for the particles, GFP tagged proteins for the cell). We use superparamagnetic nanoparticles which were heated by radio-frequency magnetic fields in a custom built set-up permitting simultaneous fluorescence imaging. We studied the differences in the nature of heat dissipation between particles bound to cell membranes and particles in suspension, undergoing Brownian motion. A comparative study of the geometric effects arising from particle arrangement and placement within the cells have also been studied. We find local heating to be strongly dependent on geometric particle arrangement, as well as on the relaxation mechanism of the nanoparticles, Brownian or Neel relaxation. To aid the understanding of the theoretical aspects of nanoscale heat transfer we incorporate simulations of the nanoparticle heating mechanism and heat dissipation in our study. Together, these results provide insights on efficient experimental designs for biomedical application of nanoparticle heating in-vivo.

A Robust Finite Element Modeling Approach to Conjugate Heat Transfer in Flexible Elastic Tubes and Tube Networks

In this work, heat transfer between fluid flow in elastic tubes and external environment is modeled using a robust finite element approach. The transport of energy is coupled to fluid flow that is linked to the pressure and cross-sectional area variations of the tube. The novel model developed is applied to flow and heat transfer in elastic tubes with different geometric and material properties. The effects of reflections due to discontinuities and bifurcations in the tubes are also investigated. To determine the heat transport by conduction in the elastic walls, a radial heat conduction model is also incorporated. The coupled flow equations are solved using the locally conservative Galerkin finite element method, which provides an explicit element-wise conservation of fluxes. Several simulations are performed for different parametric variations to understand the relevant aspects of heat transfer in flexible elastic tubes. The results show that small temperature fluctuations are possible, inline with the pulsatile flow boundary conditions. It is also observed that increased flexibility of tubes leads to better heat transfer between the fluid and the wall. The results clearly indicate that any flow reflections also increase the heat transfer between the fluid and the wall.

Heat transfer and acoustic study of impinging synthetic jet using diamond and oval shape orifice

Synthetic jet is a novel flow phenomenon potentially useful for cooling of electronic devices. The heat transfer and acoustic characteristics of impinging synthetic jet are investigated experimentally using different configurations of diamond and oval shape orifices with same hydraulic diameter. The experiments are conducted at an excitation frequency of 100–250 Hz. The ratio of axial distance between the heated copper block and orifice plate to orifice diameter (z/d) is in the range of 0–16. The average heat transfer coefficient and the sound pressure level (SPL) are obtained for different configurations of the orifice. The maximum value of the average heat transfer coefficient using diamond and oval shape orifices is found to be 17% and 7% higher respectively as compared to circular orifice of the same hydraulic diameter at 200 Hz. For all the configurations of oval shape orifice experimented in the present study, Nusselt number gets maximized at the same location. However, no such behaviour is observed in case of diamond shape orifice. The sound pressure level (SPL) obtained in case of diamond and oval shape orifices is less by 7 dB as compared to circular orifice at an excitation frequency of 200 Hz. The oval shape orifice is found to perform better in terms of heat transfer and acoustic aspect. SPL obtained for all the orifice shapes considered in this study is less than 40 dB at 100 Hz which is acceptable as per the United States Environmental Protection Agency (USEPA) and therefore synthetic jet with these orifice shapes can practically be employed for cooling applications without any further efforts for noise reduction.

Considering In-Tube Crude Oil Boiling in Assessing Performance of Preheat Trains Subject to Fouling

Oil refinery preheat trains can exhibit unwanted two-phase flow behavior. An example is boiling of crude oil inside heat exchangers, when the local pressure is not high enough to keep crude in a liquid state. This often arises when the pump is undersized. Understanding the two-phase behavior and assessing the boiling heat transfer coefficients would result in a better prediction and estimation of exchanger fouling. Where single-phase modeling is used under boiling conditions, the anomalous behavior leads to unrealistic estimates of fouling resistance, and can severely underpredict the increased pressure drop and consequent loss of crude throughput. There is little public information on fouling in two-phase flows as laboratory experiments are very costly, despite the importance of this in refinery heat exchangers and furnaces. Indeed, the importance of crude boiling is likely to increase as lighter crudes such as shale oils are processed. These lighter crudes are often blended with heavier crudes to maintain an appropriate refining average density. This paper consists of two sections. The first section uses industrial monitoring data to illustrate fouling behavior for a heat exchanger that undergo both boiling and fouling. The second section discusses simulations to evaluate thermohydraulic behavior when the crude undergoes boiling. The analysis requires coupled heat transfer, and hydraulic and surface fouling aspects; a commercial preheat train network simulator, SmartPM, was used for this study.

Natural Convection from a Heated Sphere in Bingham Plastic Fluids

Laminar natural convection from an isothermal sphere immersed in cold Bingham plastic media has been investigated numerically over wide ranges of the pertinent dimensionless parameters (Rayleigh number, 102 ≤ Ra ≤ 106; Prandtl number, 10 ≤ Pr ≤ 100; and Bingham number, 0 ≤ Bn ≤ 104). Extensive results on the flow and heat-transfer aspects are presented in terms of the streamlines and isotherm contours in the close proximity of the sphere; morphology of the yielded and unyielded regions; and the local and average Nusselt number as functions of the Rayleigh number, Prandtl number, and Bingham number. For a given value of the Rayleigh number, there exists a limiting Bingham number beyond which no convection occurs and heat is transferred from the sphere to the fluid only by conduction. Broadly, fluidlike regions are seen to gradually diminish with the increasing Bingham number, and eventually the flow field is completely frozen, corresponding to the fully plastic limit. Finally, the present numerical results of the average Nusselt number are correlated via a simple expression, thereby enabling the interpolation of the present results for the intermediate values of the parameters.

Specific aspects of heat transfer in solid tetrahydrofuran

The isochoric thermal conductivity of solid tetrahydrofuran C4H8O (a cyclic hydrocarbon with pseudorotational motion of molecules) was studied using samples of different density in the temperature range 120−180 K. For all the samples, the isochoric thermal conductivity was found to decrease with temperature following a much weaker dependence than 1/T. The experimental data were interpreted in terms of the Debye model of thermal conductivity considering heat transfer by both low-frequency phonons and “diffuse” modes. No significant contribution to the thermal conductivity by pseudorotation motion of the molecules was found.

Mixed Convection in Power-Law Fluids from a Heated Semicircular Cylinder: Effect of Aiding Buoyancy

Laminar aiding-buoyancy mixed-convection heat transfer in power-law fluids from a heated semicircular cylinder with its flat base facing the oncoming flow was studied in this work. The coupled momentum and energy equations were solved numerically over the following ranges of conditions: Richardson number (0 ≤ Ri ≤ 2), Reynolds number (0.1 ≤ Re ≤ 25), Prandtl number (1 ≤ Pr ≤ 100), and power-law index (0.3 ≤ n ≤ 1.8). Extensive results on the velocity and temperature fields are presented and discussed in terms of the streamline and isotherm contours adjacent to the heated object. Similarly, the distribution of the pressure coefficient and the local Nusselt number along the surface of the semicircular cylinder are analyzed to delineate the influence of the relevant dimensionless parameters listed above. Next, the pressure and total drag coefficient and surface average values of the Nusselt number describe the momentum and heat transfer aspects at the gross macroscopic level. Both the pressure and total drag coefficient exhibit the expected dependence on Richardson number, Prandtl number, and Reynolds number. Similarly, the average Nusselt number bears a positive dependence on each Richardson number, Prandtl number, and Reynolds number. Broadly, under otherwise identical conditions, heat transfer is augmented in shear-thinning fluids (n < 1) whereas it is somewhat impeded in shear-thickening fluids (n > 1) with reference to that in Newtonian fluids. Finally, the present numerical results of the average Nusselt number (recast in terms of the Colburn j H factor) were correlated as functions of the modified Reynolds number, Prandtl number, and power-law index thereby enabling its prediction in a new application.

Thermodynamic aspects of therapeutic hypothermia

Therapeutic hypothermia (TH) is an important treatment for post-cardiac arrest syndrome. Despite its widespread practice, only limited data describe the thermodynamic aspects of heat transfer during TH. This paper reviews the principles of human body heat balance and provides a conceptual model for characterizing heat exchange during TH. The model may provide a framework for computer simulation for improving training in or clinical methods of TH.

Ultrafast cooling of a hot moving steel plate by using alumina nanofluid based air atomized spray impingement

The objective of the present work is to examine the heat transfer aspect of alumina nanofluid based air atomized spray impingement on a hot moving steel plate with an initial temperature well above the Leidenfrost point. The influence of surfactants in increasing the effectiveness of nanofluid as a heat transfer media has also been investigated. The experimental study has been conducted with four different types of coolants namely water, water-alumina, water-alumina-SDS and water-alumina-tween20. The thermo-physical properties (viscosity and thermal conductivity) of the coolants have been measured as they affect the heat transfer rate. The heat transfer result indicates that enhanced cooling rates are obtained using nanofluids as compared to that of water.

ChemInform Abstract: Properties of Circulating Fluidized Beds (CFB) Relevant to Combustion and Gasification Systems

AbstractReview: 83 refs.

Hall effects on peristaltic flow of couple stress fluid in an inclined asymmetric channel

Effect of Hall current on peristaltic transport of couple stress fluid in an inclined asymmetric channel is investigated. An incompressible fluid is conducting in the presence of inclined magnetic field. Problem formulation is developed through heat and mass transfer aspects. Soret and Dufour effects are present. Long wavelength and low Reynolds number approaches are adopted. Closed form expression for longitudinal velocity, pressure gradient, temperature and concentration are presented. Important results reflecting the influence of embedded parameters in the problems have been pointed out. Pumping and trapping phenomena are especially focused.

The Mechanism of Moisture Transport in a Gel Cylinder

In drying of gels, the analysis of the process must encompass both heat and mass transfer aspects as well as deformation, usually very strong, of the material in the process. Deformation is almost always assumed to be linearly dependent on moisture content without considering an internal stress or pressure build-up. Since there is a great commercial interest in production of non-deformed dry gels by convective ambient pressure drying, it was interesting to investigate the behavior of such materials in drying. Convective drying would eliminate the expensive supercritical. In this work several types of gels were synthesized and dried convectively in a lab scale. Since drying induces the internal stress formation responsible for shrinkage and cracking, great care was taken to measure deformation and internal pressure development in gel during drying. Cylindrical gel samples 35 mm in diameter were dried convectively in air at 30, 40, and 50 °C. The hydrogels were made of PVP, agar-agar, and silica. Different types of behavior of the gels during drying was observed: almost uniform deformation in the case of PVP, case-hardening for agar-agar gels, and cracking for silica hydrogels. This behavior directly results from different mechanical properties of the investigated gels. It was also observed that the internal pressure changes from initially overpressure to underpressure in the course of drying which is also a result of internal stress build up. The experimental data may be used for validation of mechanical models of drying of non-linear visco-elastic solids.

Forced convective heat transfer in boundary layer flow of Sisko fluid over a nonlinear stretching sheet.

The major focus of this article is to analyze the forced convective heat transfer in a steady boundary layer flow of Sisko fluid over a nonlinear stretching sheet. Two cases are studied, namely (i) the sheet with variable temperature (PST case) and (ii) the sheet with variable heat flux (PHF case). The heat transfer aspects are investigated for both integer and non-integer values of the power-law index. The governing partial differential equations are reduced to a system of nonlinear ordinary differential equations using appropriate similarity variables and solved numerically. The numerical results are obtained by the shooting method using adaptive Runge Kutta method with Broyden’s method in the domain. The numerical results for the temperature field are found to be strongly dependent upon the power-law index, stretching parameter, wall temperature parameter, material parameter of the Sisko fluid and Prandtl number. In addition, the local Nusselt number versus wall temperature parameter is also graphed and tabulated for different values of pertaining parameters. Further, numerical results are validated by comparison with exact solutions as well as previously published results in the literature.

Spheroids in Viscoplastic Fluids: Drag and Heat Transfer

In the present work, the forced convection momentum and heat transfer aspects of isothermal spheroidal particles (both prolates and oblates) in Bingham plastic fluids have been numerically investigated in the steady axisymmetric flow regime. Extensive results on the detailed structures of the flow and temperature fields are presented and analyzed in terms of the streamline and isotherm contours, and the yield surfaces as well as their dependence on the pertinent influencing parameters, namely, Reynolds number (1 ≤ Re ≤ 100), Prandtl number (1 ≤ Pr ≤ 100), and Bingham number (0 ≤ Bn ≤ 100) are delineated. Five values of aspect ratio, e = 0.2 and 0.5 (oblates) and e = 2 and 5 (prolates), and the limiting case of a sphere, i.e., e = 1, are considered here to elucidate the effect of shape on both drag and Nusselt number values. Broadly, for a given shape (value of e), drag shows the classic inverse dependence on the Reynolds number and a positive correlation with the Bingham number. Similarly, the mean Nusselt number bears a positive dependence on each of these parameters, Re, Pr, and Bn, due to the sharpening of the temperature gradient in the thin thermal boundary layer. The present drag results have been correlated via the use of a modified Reynolds number whereas the heat transfer results have been consolidated in terms of the Colburn jH factor, thereby enabling their prediction in a new application.