Unsteady flow and heat transfer on an accelerating surface with blowing or suction in the absence and presence of a heat source or sink

The problem of unsteady flow and heat transfer in the laminar boundary layer on a linearly accelerating surface with suction or blowing in the absence and presence of a heat source or sink is considered. The governing partial differential equations for this investigation are transformed into the non-dimensional equations by using pseudo-similarity time and pseudo-similarity coordinate. The resulting two points boundary-value problem is solved numerically by the central finite difference method associated with Newton's iteration from the initial stage (ξ=0) to a steady state (ξ=1) completely. A parametric study is performed to illustrate the effects of Prandtl number, power-law surface temperature (PLST) or power-law heat flux (PLHF), heat sink or heat source, and suction or blowing parameter on the dynamic velocity and temperature fields as well as the transient development of the skin-friction coefficients and the Nusselt number. These results are depicted graphically to display special aspects of unsteady flow and heat transfer characteristics in all time.     

Experimental investigation of flow resistance and convection heat transfer of CO2 at supercritical pressures in a vertical porous tube

Experiments were conducted to measure the convection heat transfer in a vertical porous tube with particle diameters of 0.2–0.28 mm at supercritical pressures. The local heat transfer coefficients, fluid bulk temperatures and wall temperatures were measured to investigate the influence of the inlet fluid temperature, pressure, heat flux and flow direction on the convection heat transfer in the porous tube. The measured friction factors in a heated tube are much larger than those predicted using the Aerov-Tojec correlation for both upward and downward flows. Therefore, two new correlations are presented for upward and for downward flows to predict the friction factors of supercritical pressure CO₂ in heated porous tubes. The experimental results also show that the inlet temperature, pressure and heat flux all significantly influence the convection heat transfer. When the inlet temperature (T₀) is higher than the pseudocritical temperature (T_pc), the local heat transfer coefficients are much less than those when the inlet temperature is lower than the pseudocritical temperature. The convection heat transfer coefficients are found to vary nonlinearly with heat flux. For T₀ < T_pc, the local heat transfer coefficients along the porous tube have a maximum for upward flow and have a peak value for downward flow when the local fluid bulk temperatures are near T_pc and the wall temperatures are slightly higher than T_pc. The different variations of the local heat transfer coefficients along the porous tube for upward and downward flows are attributed to the effect of buoyancy. However, when the wall temperatures are much higher than T_pc, the local heat transfer coefficients along the porous tube decrease continuously for both upward and downward flows.     

NONSIMILAR SOLUTIONS FOR HEAT TRANSFER IN WALL JET FLOWS

An analysis is presented to investigate the flow and heat transfer characteristics of a laminar plane wall jet with non-isothermal wall as well as uniform suction and blowing at the surface and a laminar cylindrical wall jet. The approach used is local nonsimilarity method, wherein, the nonsimilanty terms appearing in the momentum and energy equations are retained and simplifications are introduced only in the auxiliary system of equations. To insure the accuracy of the results, solutions are obtained for three levels of truncation of the governing equations. For the case of plane wall jet problem, both a series solution as well as local nonsimilarity solution have been given and the agreement between the two is found to be very good. For the cylindrical wall jet problem, the results obtained by the local nonsimilarity approach in the present paper have been compared with the series solution results. Numerical results for the wall shear stress, velocity distribution, wall heat transfer rate and temperature field are presented.     

EFFECTS OF SURFACE TENSION ON LAMINAR FILMWISE CONDENSATION ON A HORIZONTAL PLATE IN A POROUS MEDIUM WITH SUCTION AT THE WALL

The problem of two-dimensional, steady-state film condensation on an isothermal finite-size horizontal plate embedded in a porous medium is studied for the case in which the plate faces upward into a region of dry saturated vapor. Due to surface tension effects, a two-phase zone is formed between the liquid film on the horizontal plate and the vapor zone. The effects of surface tension are shown to reduce the thickness of the liquid film and hence to increase the heat transfer performance of the horizontal plate. Furthermore, the results show that the dimensionless heat transfer coefficient depends on the Darcy number Da, the Jacob number Ja, the effective Rayleigh number Ra_e, the effective Prandtl number Pr_e, the wall suction parameter Sw, and the surface tension parameter Bo_c. When the surface tension effects are neglected and there is no suction at the wall, a closed-form correlation for the Nusselt number can be established.     

满液式蒸发器中螺旋扁管的池沸腾传热

对螺旋扁管在满液式蒸发器中的池沸腾传热进行了实验研究。螺旋扁管由外径为15.88 mm的圆形满蒸管加工而得,其外径、壁厚以及长度分别为19.50 mm×11.28 mm、1.09 mm和3310 mm。通过实验研究了管程Reynolds数Re_i、制冷剂饱和温度T_sat、管壁过热度T_sup以及热通量q_b对于池沸腾传热性能的影响。结果表明,随着Re_i、T_sat和q_b的增加,螺旋扁管满液式蒸发器以及原有满液式蒸发器的沸腾传热性能都随之增强,而随着T_sup的增加,两者的沸腾传热性能却呈下降趋势。同时,对装有两种满液式蒸发器的螺杆式冷水机组分别进行了测试,结果表明在换热量相同的条件下,螺旋扁管满液式蒸发器比原有蒸发器的总传热系数提高了15%左右,证明螺旋扁管满液式蒸发器在螺杆式冷水机组中的应用是可行的。     

Numerical simulation of natural convection in a square enclosure filled with nanofluid using the two-phase Lattice Boltzmann method

Considering interaction forces (gravity and buoyancy force, drag force, interaction potential force, and Brownian force) between nanoparticles and a base fluid, a two-phase Lattice Boltzmann model for natural convection of nanofluid is developed in this work. It is applied to investigate the natural convection in a square enclosure (the left wall is kept at a high constant temperature (T_H), and the top wall is kept at a low constant temperature (T_C)) filled with Al₂O₃/H₂O nanofluid. This model is validated by comparing numerical results with published results, and a satisfactory agreement is shown between them. The effects of different nanoparticle fractions and Rayleigh numbers on natural convection heat transfer of nanofluid are investigated. It is found that the average Nusselt number of the enclosure increases with increasing nanoparticle volume fraction and increases more rapidly at a high Rayleigh number. Also, the effects of forces on nanoparticle volume fraction distribution in the square enclosure are studied in this paper. It is found that the driving force of the temperature difference has the biggest effect on nanoparticle volume fraction distribution. In addition, the effects of interaction forces on flow and heat transfer are investigated. It is found that Brownian force, interaction potential force, and gravity-buoyancy force have positive effects on the enhancement of natural convective heat transfer, while drag force has a negative effect.     

THE EFFECT OF FREE STREAM CONCENTRATION ON HEAT AND BINARY MASS TRANSFER WITH THERMODYNAMIC COUPLING IN CONVECTION ON A ROTATING DISC†

Simultaneous heat and mass transfer, which arises from injecting a gas (helium or hydrogen) from or through the solid surface into a flowing external stream, has been studied for a rotating disc geometry. The effects of concentration levels of the injected gas in the external stream on the thermodynamic coupling in the presence of centrifugal force have been investigated over a wide range of T_w/T_e.

Boundary layer equations for heat and mass transfer were solved numerically. Exact and linearized approximate solutions were obtained. The results have shown that the thermal diffusion effect on mass transfer becomes increasingly important as the free stream concentration increases and as T_w/T_e departs from unity. The diffusion thermo effect on heat transfer was found to be the most important when the free stream concentration is zero and as T_w/T_e approaches unity.     

A Method for Determining Bulk Density,Material Density,and Porosity of Melter Feed During Nuclear Waste Vitrification

Glassmelting efficiency largely depends on heat transfer to reacting glass batch (melter feed), which in turn is influenced by the bulk density (ρ_b) and porosity (?) of the reacting feed as functions of temperature (T). Neither ρ_b(T) nor ?(T) functions are readily accessible from direct measurements. For the determination of ρ_b, we monitored the profile area of heated feed pellets and calculated the pellet volume using numerical integration. For the determination of ?, we measured the material density of feeds quenched at various stages of conversion via pycnometry and then computed the feed density at heat‐treatment temperature using thermal expansion values of basic feed constituents.     

A NEW CLASS OF SIMILARITY SOLUTIONS OF AN UNSTEADY ELECTRICALLY CONDUCTING FREE-FORCED CONVECTIVE FLOW IN A VERTICAL POROUS SURFACE WITH DUFOUR AND SORET EFFECTS

The 2-D unsteady magnetohydrodynamic free-forced convective boundary layer flow of a viscous incompressible fluid is studied numerically taking into account heat and mass transfer. The fluid is subjected to uniform heat and mass fluxes embedded in a porous medium by the presence of coupled Dufour and Soret effects. A new class of similarity equations has been obtained by introducing a time-dependent length scale and a corresponding similarity variable. The resulting equations are then integrated numerically using the Nachtsheim-Swigert shooting iteration technique along with the sixth-order Runge-Kutta integration scheme. By developing locally similar solutions of the fluid flow, the behavior of the velocity, temperature, and concentration fields as well as the rate of heat transfer, wall temperature gradient, rate of mass transfer, and skin friction coefficient have been investigated. The effects of Grashof number (Gr), modified Grashof number (Gm), combined effects of the porous and magnetic parameter (S), suction/injection parameter Fw, Brinkman number (Br), Soret number (Sr), and Dufour number (D_f) have been observed on the flow field and discussed.     

The effect of free stream concentration on heat and binary mass transfer with thermodynamic coupling in forced convection on a flat plate

Simultaneous heat and mass transfer, which arises from injecting a gas into a flowing external stream, has been studied analytically for a flat plate geometry. The studies are centered around the effect of free stream concentration of the injected gas on the thermodynamic coupling, over a wide range of T_w/T_e.Exact and linearized approximate solutions were obtained numerically. It was found that the thermal diffusion effect on mass transfer becomes increasingly important as the free stream concentration increases and T_w/T_e departs from unity. On the other hand the diffusion thermo effect on heat transfer was found to be the most important when the free stream concentration is zero and as T_w/T_e, approaches unity.     

SLIP EFFECT ON DIFFUSION OF CHEMICALLY REACTIVE SPECIES IN BOUNDARY LAYER FLOW OVER A VERTICAL STRETCHING SHEET WITH SUCTION OR BLOWING

In the present investigation, we study the effects of slip boundary condition on the diffusion of chemically reactive species in steady boundary layer flow of viscous incompressible fluid over a vertical stretching sheet with suction or blowing. The first-order chemical reaction is considered and wall concentration varies linearly along the sheet. The self-similar equations are obtained using similarity transformations and are solved numerically using shooting method. Our study reveals that due to the increase of diffusion parameter and blowing, the velocity increases, and it decreases with suction, Schmidt number, and reaction rate parameter. Importantly, for increase of slip parameter, the boundary layer thickness increases. In contrast, the concentration at a point increases only for increasing slip and blowing, while it decreases for increase of all other parameters.     

Evaluation of heat transfer parameters in packed beds with cocurrent downflow of liquid and gas

The results of an experimental investigation on heat transfer from a packed bed with cocurrent gas–liquid downflow to the wall are presented and analyzed in this contribution. The measurements cover the range of operating variables corresponding to the so-called trickle regime in beds presenting aspect ratios (tube to particle diameter ratio) from 4.67 to 34.26. Water and air were employed as model fluids. The heat transfer process was first analyzed by means of a two-dimensional pseudohomogeneous plug-flow model with two parameters, the effective radial thermal conductivity (k_er) and the wall heat transfer coefficient (h_w). k_er is well correlated with liquid and gas Reynolds numbers and particle diameter, except for the lowest experimental aspect ratio (4.67). Instead, a meaningful correlation of h_w stands only for aspect ratios larger than 15. These results are analyzed and the evidence points out to sustain the hypothesis that the model fails at low aspect ratios because an apparent contact resistance (1/h_w) can no longer accommodate the effects of significant fluid bypassing and finite size of the near-wall region. The experimental set of data were also used to develop a correlation for the overall heat transfer coefficient (h_T), which can be employed satisfactorily to predict heat transfer rates in the whole range of variables here investigated.     

MEASUREMENT OF COEFFICIENTS FOB SIMULTANEOUS HEAT AND MASS TRANSFER IN FOOD PRODUCTS

ABSTRACT

Two experimental devices were designed and built to determine four coefficients K_T K_M D_M D (or δ_T = D_T / D_M occurring in simultaneous heat and mass transfer equations, where,K _T and D_M are thermal conductivity and moisture diffcusivity respectively, D_T ( or δ _T is temperature gradient Induced moisture migration coefficient and K_M is moisture gradient Induced heat transfer coefficient. Three food materials, i.e. potato, bread dough and bread, were tested. From this study, it was found that the value of 5 was higher for low density food materials, such as bread, than for high density materials, such as potato. The coefficient & measures moisture migration contribution due to temperature gradient within the material. The average values of δ _T for potato, bread dough and bread were 0.0014, 0.0059 and 0.0127 per °C, respectively. The contribution of temperature gradient to the overall moisture migration is negligible In high density materials. However, this contribution may be important in the moisture migratlon analysls for low density materials. The moisture gradient induced heat transfer coefficient % as found to be negligible for the materials tested in this study     

Numerical studies of the flash pyrolysis of cellulose

Numerical simulations reveal that the time required for rapid pyrolysis of cellulosic biopolymer particles (diameter ? 0.5 mm) is composed of a heatup time t_h and a devolatilization time t_d. Approximations of t_h and t_d are given by the time required for radiative and convective heat transfer to raise the particle's temperature to a pyrolysis temperature T_p (‘sensible’ heat requirement) and to provide the endothermic heat of reaction ΔH (‘latent’ heat requirement). The value of T_p is specified by the chemical rate law governing pyrolysis. Recently reported rate laws place a practical upper limit on T_p of ≈ 500 °C and provide for the complete vaporization of cellulose at high heating rates. Strong analogies exist between the time required for a given power input to provide both the sensible and latent heat requirements of a solid undergoing a phase change and the time required for heat transfer to effect cellulose pyrolysis. Correction factors reported here permit an accurate estimate of the time required for cellulose pyrolysis, based on the times required for the rate of heat transfer (power input) to provide the ‘sensible’ and ‘latent’ heats of pyrolysis at the (‘melting point’) temperature T_p.     

Rheological behavior of ethylene–octene copolymer vulcanizates: Effect of blowing agent and precipitated silica filler

An experimental study of the rheological behavior of ethylene–octene copolymer vulcanizates in extrusion containing blowing agent has been carried out. The cell morphology development has been studied through a scanning electron microscope. Rheological properties of unfilled and precipitated silica‐filled systems with variations of blowing agent, extrusion temperature, and shear rate have been studied by using a Monsanto processibility tester (MPT). The total extrusion pressure (P_T), apparent shear stress (τ_wa), apparent viscosity (η_a), and die swell (%) of the unfilled and silica‐filled compounds have been determined by using MPT. The effect of blowing agent (ADC) on the rheological properties of the vulcanizates has also been investigated. There is a reduction of stress and viscosity with blowing agent loading. It was observed that the incorporation of a blowing agent led to decreased shear thinning behavior resulting in an increase in power law index. The viscosity reduction factor (VRF) of unfilled vulcanizates is found to be dependent on the concentration of the blowing agent, shear rate, and temperature, whereas VRF of silica‐filled vulcanizates is found to be dependent on shear rate, temperature, and blowing agent concentration. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1132–1138, 2003     

Natural convection heat transfer enhancement through latent heat transport in vertical parallel plate channel flows

A numerical analysis is carried out to investigate the effects of latent heat transfer, in connection with the vaporization of a liquid film, on natural convection heat transfer in a vertical parallel plate channel. Major nondimensional groups identified are Gr_T, Gr_M, Pr and Sc. Results for Nusselt and Sherwood numbers are specifically presented for the air-water and air-ethanol systems under various heating conditions to illustrate the heat transfer enhancement through latent heat transfer during the evaporation processes. Considerable enhancement in heat transfer due to the exchange of latent heat was clearly demonstrated.     

Wall heat transfer to chemical reactors

The effects of chemical reactions on wall heat transfer coefficients in packed bed reactors operating at equilibrium or near equilibrium conditions are explored by means of one-dimensional and two-dimensional models. Both approaches suggest using Damköhler number or ratios of reacting to frozen heat capacity as parameters in heat transfer correlations. Available data for the methane-steam reforming reactors were used to establish a suitable correlation for use in thermal design of reacting beds of solids, i. e. N_Nu = 0.195 N_Re^0.8 exp. (0.21 σ) This correlation can be applied with reasonable accuracy for beds which are 3 to 5-in. in diameter and bed to particle diameter ratios from 5 to 10. A review of available correlations for wall coefficients obtained under non-reacting packed bed conditions is also presented.     

Investigation of thermoplastic elastomer (TPE) foaming process using blowing agent by rheological and morphological methods

The effects of a chemical blowing agent (CBA) or an encapsulated physical blowing agent (PBA) on morphological development in ethylene octene copolymer (EOC) matrix using dicumyl peroxide (DCP) as a curing agent were investigated by rheological, mechanical, and morphological methods. Temperature ramp tests were carried out to understand curing and foaming processes. Curing temperature (T_cure) was determined as the crossover temperature where storage modulus G′ coincided with loss modulus G′′ in the rheological point of view. For the CBA, T_cure increased with increasing CBA concentration, whereas for the PBA, T_cure decreased with increasing PBA concentration. Other critical temperatures T_1st, T_2nd by foaming process were determined using the axial normal force. With these critical temperatures (T_cure, T_1st, T_2nd), curing and foaming mechanisms can be estimated. Simultaneously, volume expansions of samples were observed with camera. Morphology and mechanical analysis were conducted on fully cured and foamed ECP (is defined as EOC with DCP) with blowing agent. ECP with the CBA exhibited an irregular open-cell structure, whereas when produced using the PBA, it formed a regular closed-cell structure. Specific tensile strength tended to increase with increasing PBA concentration but as blowing agent concentration increased elongation at break decreased. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47358.     

Simulation of forced convection in a channel with nanofluid by the lattice Boltzmann method

This paper presents a numerical study of the thermal performance of fins mounted on the bottom wall of a horizontal channel and cooled with either pure water or an Al₂O₃-water nanofluid. The bottom wall of the channel is heated at a constant temperature and cooled by mixed convection of laminar flow at a relatively low temperature. The results of the numerical simulation indicate that the heat transfer rate of fins is significantly affected by the Reynolds number (Re) and the thermal conductivity of the fins. The influence of the solid volume fraction on the increase of heat transfer is more noticeable at higher values of the Re.     

Heat transfer to horizontal tubes immersed in a fluidized-bed combustor

Experiments are carried out to measure the heat transfer rates to water-cooled horizontal tubes (D_T = 25.4 mm) immersed in an atmospheric fluidized-bed combustor burning North Dakota lignite. Silica sand, with average diameter ranging from 0.544 mm to 2.335 mm, is used as bed material. The tests are conducted at average bed temperatures ranging from 587 to 1205 K. For bed temperatures less than 950 K, the bed is heated by a propane gas heater. The superficial velocity is varied from 0.73 to 2.58 m/s. Thus, the effect of bed temperature (T_B = 587 – 1205 K), particle size (d̄_p = 0.544 – 2.335 mm), and fluidizing velocity (U = 0.73 – 2.58 m/s) on the heat transfer rate to horizontal tube immersed in a fluidized-bed combustor (0.45 m × 0.45 m) is investigated. Among the existing correlations, those correlations proposed by Glicksman and Decker, Zabrodsky et al., Catipovic et al., Grewal, and Bansal et al. are found to predict the present data quite well, when the contribution due to radiation is included. The radiative heat transfer is estimated as the difference between the heat transfer to oxidized boiler tube and gold-plated tube. The relative contribution of radiation is found to be 13% for a bed of sand particles (d̄_p = 0.9 mm) operating at 1087 K.