Three dimensional unsteady convection and mass transfer flow through porous medium

The problem of three dimensional unsteady convection flow through a porous medium, with effect of mass transfer bounded by an infinite vertical porous plate is discussed, when the suction at the plate is transverse sinusoidal and the plate temperature oscillates in time about a constant mean. Assuming the free stream velocity to be uniform, approximate solutions are obtained for the flow field, the temperature field, the skin-friction and the rate of heat transfer. The dependence of solution on Pr (Prandtl number), Gr (Grashof number based on temperature), Gc (modified Grashof number based on concentration difference), Sc (Schimdt number), the frequency and the permeability parameter is also investigated.     

Heat transfer analysis according to condensation flow structures in a minichannel

An experimental investigation of complete condensation flow is undertaken for a range of mass flow rates between 3.4 and 13.8 kg/m² s. The associated flow regimes are visualized using an ombroscopic technique. Two major flows are observed (with or without release of bubbles). A critical value of the mass flow rate is obtained at the transition between these two regimes. The visualization also enables a local parameter to be determined: the void fraction. The influence of the presence of a bubbly zone is highlighted by the heat transfer and pressure drops. Finally, the dependence of the critical value of the mass flow rate on the temperature of the secondary flow is obtained.     

Heat transfer characteristics in a horizontal swirling fluidized bed

An innovative horizontal swirling fluidized bed (HSFB) with a rectangular baffle in the center of an air distributor and three layers of horizontal secondary air nozzles located at each corner of fluidized bed was developed. Experiments on heat transfer characteristics were conducted in a cold HSFB test model. Heat transfer coefficients between immersed tubes and bed materials in the HSBF were measured with the help of a fast response heat transfer probe. The influences of fluidization velocity, particle size of bed materials, measurement height, probe orientation, and secondary air injection, etc. on heat transfer coefficients were intensively investigated. Test results indicated that heat transfer coefficients increase with fluidization velocity, and reach their maximum values at 1.5-3 times of the minimum fluidization velocity. Heat transfer coefficients are variated along the circumference of the probe, and heat transfer coefficients on the leeward side of the probe are larger than that on the windward side of the probe. Heat transfer coefficients decrease with increasing of measurement height; heat transfer coefficients of the longitudinal probe are larger than that of the transverse probe. The proper secondary air injection and particle size of bed materials can generate a preferred hydrodynamics in the dense zone and enhance heat transfer in a HSFB.     

Heat transfer characteristics of a two-phase closed thermosyphons using nanofluids

The heat transfer characteristics of the heat transfer devices can be done by changing the fluid transport properties and flow features of working fluids. In the present study, therefore, the heat transfer characteristics of two-phase closed thermosyphon (TPCT) with iron oxide-nanofluids are presented. The TPCT is fabricated from the copper tube with the outer diameter and length of 15, 2000 mm, respectively. The TPCT with the de-ionic water and nanofluids (water and nanoparticles) are tested. The iron oxide nanoparticles with mean diameter of 4-5 nm were obtained by the laser pyrolysis technique and the mixtures of water and nanoparticles are prepared using an ultrasonic homogenizer. Effects of TPCT inclination angle, operating temperature and nanoparticles concentration levels on the heat transfer characteristics of TPCT are considered. The nanoparticles have a significant effect on the enhancement of heat transfer characteristics of TPCT. The heat transfer characteristics of TPCT with the nanofluids are compared with that the based fluid.     

Heat transfer during transient compression: Measurements and simulations

Experiments have been performed to assess the utility of unsteady one-dimensional heat conduction modelling for the calculation of heat losses during a free piston compression process. Heat transfer measurements have been obtained within a gun tunnel barrel using surface junction thermocouple instrumentation. The gun tunnel was operated with a relatively heavy piston such that the shock waves induced by the piston motion were weak. Peak heat transfer values are estimated reasonably well by the unsteady one-dimensional model. However, overall quantitative agreement between the measurements and calculations has not been achieved at this stage, principally because the development of turbulent heat transport was not properly modelled. Received 21 September 2001 / Accepted 11 March 2002 – Published online 11 June 2002     

Heat transfer characteristics and Nusselt number correlation of turbulent pulsating pipe air flows

Heat transfer characteristics to turbulent pulsating pipe flows under a wide range of Reynolds number and pulsation frequency were experimentally investigated under uniform heat flux condition. Reynolds number was varied from 8462 to 48540 while the frequency of pulsation ranged from 1 to 29.5 Hz. The results showed that the relative mean Nusselt number is strongly affected by both pulsation frequency and Reynolds number. Enhancements in mean Nusselt number of up to 50% were obtained at medium pulsation frequency between 4.1 and 13.9 Hz for Reynolds number range of 8462 to 14581. An enhancement of up to 50% in mean Nusselt number was obtained at high pulsation frequency range between 13.9 and 29.5 Hz, specially as Reynolds number is close to 15000, while a reduction was observed at higher Reynolds number more than 21200. This reduction, at high Reynolds number, increased as pulsation frequency increased. Also, there was a reduction in mean Nusselt number of up to 20% that obtained at low pulsation frequency range between 1 and 4.1 Hz for Reynolds number range of 8462 to 48543. A significant reduction in mean Nusselt number of up to 40% was obtained at medium pulsation frequency between 4.1 and 13.9 Hz for Reynolds number range of 21208 to 48543. Empirical equations have been developed for the relative mean Nusselt number that related to Reynolds number and dimensionless frequency with about uncertainty of 10% rms.The support of both King Fahd University of Petroleum and Minerals and Cairo University for this research is acknowledged.     

Heat transfer during bubble formation

The flow around a gas bubble forming at a submerged orifice without vaporisation is analysed both with and without heat transfer taking place from the bubble surface. The results of experiments are given in which high-speed photography was used to study air bubbling through four different sizes of orifice into water, for the isothermal and diabatic case. A numerical solution of the dimensionless equations using Hamming's modified predictor-corrector method gave excellent agreement with experimental measurements of the bubble volume-time history for both stages of growth as well as predicting accurately the transition point between the two stages. Most of the heat transfer was found to occur almost immediately after the start of bubble growth     

Heat transfer effects on hydropneumatic suspension systems

One of the problems experienced on hydropneumatic suspension systems, is the effect of temperature change on the spring characteristic, resulting in variations in spring rate and ride height. This problem can be analysed using an appropriate heat transfer model. Two major heat transfer modes are investigated, i.e. heat transfer between the gas and its surroundings and heat transfer between the damper oil and the gas. The analyses performed include the determination of the dynamic spring force characteristic, the effect of heat build-up on the spring force characteristic and the effect of heat build-up on the performance of the hydropneumatic spring and damper unit during vehicle tests. Mathematical modelling of the spring characteristic is performed by solving the energy equation for a gas in a closed container using the thermal time constant approach. The Benedict–Webb–Rubin equation of state is used for real gas behaviour. The mathematical model is verified against experimental data and good correlation is achieved. It is shown that hydropneumatic suspension systems have a significant amount of inherent damping due to heat transfer which produces no nett temperature change or heat build-up. The effect of heat build-up in the damper on the spring force characteristic is determined by laboratory tests. It is shown that heat generation in the damper, accompanied by a rise in gas temperature, has a detrimental effect on the spring force characteristic. Tests performed on experimental hydropneumatic spring and damper systems fitted to a test vehicle indicate that the effect is smaller in practice than anticipated. Heat build-up is strongly influenced by terrain roughness, vehicle speed, mission profile and damping levels.     

Semi-empirical modeling of pool boiling heat transfer in binary mixtures

Pool boiling heat transfer has been investigated for various binary mixtures, including acetone/isopropanol, water/acetone, water/methanol, water/ethanol, water/isopropanol, water/monoethanolamine, water/diethanolamine and water/triethyleneglycol as test solutions. Many correlations have been developed to predict the pool boiling heat transfer coefficient in mixtures in the past few decades, however the predicted values are not confirming. In addition, the application of many existing correlations requires some individual adjusting parameters that may be not available for every system. In this investigation, a new set of experimental data are presented. These data have been compared to major existing correlations. It is observed that the pool boiling heat transfer coefficients in mixtures are less than the ideal boiling heat transfer coefficient. A new semi-empirical model has been proposed based on the mass transfer resistance to predict the boiling heat transfer coefficient with satisfactory accuracy. The new model does not include any tuning parameter and is applicable to any given binary system. The performance of the proposed model is superior to most existing correlations.     

Experimental study on the condensation of supersonic steam jet submerged in quiescent subcooled water: Steam plume shape and heat transfer

The condensation of supersonic steam jet submerged in the quiescent subcooled water was investigated experimentally. The results indicated that the shape of steam plume was controlled by the steam exit pressure and water temperature. Six different shapes of steam plume were observed under the present test conditions. Their distribution as a function of the steam exit pressures and water temperatures was given. As the steam mass velocity and water temperature increase, the measured maximum expansion ratio and dimensionless penetration length of steam plume were in the ranges of 1.08–1.95 and 3.05–13.15, respectively. The average heat transfer coefficient of supersonic steam jet condensation was found to be in the range of 0.63–3.44 MW/m²K. An analytical model of steam plume was found and the correlations to predict the maximum expansion ratio, dimensionless penetration length and average heat transfer coefficient were also investigated.     

A tube-by-tube reduction method for simultaneous heat and mass transfer characteristics for plain fin-and-tube heat exchangers in dehumidifying conditions

This study proposed a new method, namely a tube-by-tube reduction method to analyze the performance of fin-and-tube heat exchangers having plain fin configuration under dehumidifying conditions. The mass transfer coefficients which seldom reported in the open literature, are also presented. For fully wet conditions, it is found that the reduced results for both sensible heat transfer performance and the mass transfer performance by the present method are insensitive to change of inlet humidity. Unlike those tested in fully dry condition, the sensible heat transfer performance under dehumidification is comparatively independent of fin pitch. The ratio of the heat transfer characteristic to mass transfer characteristic (h_c,o/h_d,o C_p,a) is in the range of 0.6~1.0, and the ratio is insensitive to change of fin spacing at low Reynolds number. However, a slight drop of the ratio of (h_c,o/h_d,o C_p,a) is seen with the decrease of fin spacing when the Reynolds number is sufficient high. This is associated with the more pronounced influence due to condensate removal by the vapor shear. Correlations are proposed to describe the heat and mass performance for the present plate fin configurations. These correlations can describe 89% of the Chilton Colburn j-factor of the heat transfer (j_h) within 15% and can correlate 81% of the Chilton Colburn j-factor of the mass transfer (j_m) within 20%.     

Heat transfer characteristics in a sudden expansion pipe equipped with swirl generators

This investigation is aimed at studying the heat transfer characteristics and pressure drop for turbulent airflow in a sudden expansion pipe equipped with propeller type swirl generator or spiral spring with several pitch ratios. The investigation is performed for the Reynolds number ranging from 7500 to 18,500 under a uniform heat flux condition. The experiments are also undertaken for three locations for the propeller fan (N = 15 blades and blade angle of 65°) and three pitch ratios for the spiral spring (P/D = 10, 15 and 20). The influences of using the propeller rotating freely and inserted spiral spring on heat transfer enhancement and pressure drop are reported. In the experiments, the swirl generator and spiral spring are used to create a swirl in the tube flow. Mean and relative mean Nusselt numbers are determined and compared with those obtained from other similar cases. The experimental results indicate that the tube with the propeller inserts provides considerable improvement of the heat transfer rate over the plain tube around 1.69 times for X/H = 5. While for the tube with the spiral spring inserts, an improvement of the heat transfer rate over the plain tube around 1.37 times for P/d = 20. Thus, because of strong swirl or rotating flow, the propeller location and the spiral spring pitch become influential on the heat transfer enhancement. The increase in pressure drop using the propeller is found to be three times and for spiral spring 1.5 times over the plain tube. Correlations for mean Nusselt number, fan location and spiral spring pitch are provided.     

Heat transfer characteristics of developing gaseous slip-flow in rectangular microchannels with variable physical properties

The effects of variable physical properties on the flow and heat transfer characteristics of simultaneously developing slip-flow in rectangular microchannels with constant wall temperature are numerically investigated. A colocated finite-volume method is used in order to solve the mass, momentum and energy equations in their most general form. Various channel aspect ratios are studied at different Knudsen numbers. Simulations indicate that the constant physical property assumption can result in under/over-prediction of the local friction and heat transfer coefficients depending on the problem configuration. Density and thermo-physical property variations have significant effects on predicting flow and heat transfer characteristics in the developing and fully-developed regions. The degree of discrepancy varies for different cases depending on Knudsen number, aspect ratio and the temperature difference between the channel inlet and the wall. The results suggest that even low temperature differences can alter the friction and heat transfer coefficients considerably.     

Effect of temperature gradient on Marangoni condensation heat transfer for ethanol–water mixtures

This paper experimentally studied the effect of macroscopic temperature gradient on Marangoni condensation of ethanol–water vapor mixtures under a wide range of concentrations. For each concentration, the experiment was performed at different velocities and pressures. An oblique copper block was employed to create surface temperature gradient. The results indicated that local heat flux was varied along transversal condensation surface, which was caused by surface temperature gradient. This difference in heat flux might be attributed to the variation of condensate thickness on condensation surface. In addition, a mean heat transfer coefficient was derived along transversal condensation surface. For low ethanol concentration (0.5%, 1%), the coefficient kept a high value over a relative wide range of vapor-to-surface temperature difference (<10 K) and could be augmented as much as 15% as compared with literature under similar experimental condition. Moreover, the mean heat transfer coefficient generally increased with increasing velocity or pressure for all concentrations of the ethanol–water mixtures.     

Experimental investigation on heat transfer and frictional characteristics of vertical upward rifled tube in supercritical CFB boiler

Water wall design is a key issue for supercritical Circulating Fluidized Bed (CFB) boiler. On account of the good heat transfer performance, rifled tube is applied in the water wall design of a 600 MW supercritical CFB boiler in China. In order to investigate the heat transfer and frictional characteristics of the rifled tube with vertical upward flow, an in-depth experiment was conducted in the range of pressure from 12 to 30 MPa, mass flux from 230 to 1200 kg/(m² s), and inner wall heat flux from 130 to 720 kW/m². The wall temperature distribution and pressure drop in the rifled tube were obtained in the experiment. The normal, enhanced and deteriorated heat transfer characteristics were also captured. In this paper, the effects of pressure, inner wall heat flux and mass flux on heat transfer characteristics are analyzed, the heat transfer mechanism and the frictional resistance performance are discussed, and the corresponding empirical correlations are presented. The experimental results show that the rifled tube can effectively prevent the occurrence of Departure from Nucleate Boiling (DNB) and keep the tube wall temperature in a permissible range under the operating condition of supercritical CFB boiler.     

Flow of a micropolar fluid due to a porous stretching sheet and heat transfer

The present work investigates the micropolar fluid flow due to a permeable stretching sheet and the resulting heat transfer. Unlike the existing numerical works on the flow phenomenon in the literature, the prime interest here is to analytically work out shape of the solutions and identify whether they are unique. Indeed, unique solutions are detected and presented in the exact formulas for the associated boundary layer equations. Temperature field influenced by the microrotation is also mathematically resolved in the cases of constant wall temperature, constant heat flux and Newtonian heating. To discover the salient physical features of many mechanisms acting on the considered problem, it is adequate to have the analytical velocity and temperature fields and also closed-form skin friction/couple stress/heat transfer coefficients, all as given in the current paper. For instance, the practically significant rate of heat transfer is represented by a single formula valid for all three temperature cases.     

Convection heat and mass transfer from a disk

The aim of the present study is to investigate the coupling influence of the disk rotating speed and air velocity from laboratory room on the local heat and mass characteristics from a disk in wind tunnel with the naphthalene sublimation technique. The experiments are performed at four different free stream flow velocities. From the experimental results, the correlation of Sherwood number with the coupling Reynolds number and of Nusselt number with the coupling Reynolds number are both proposed in the present work.     

Heat transfer effects on particle motion under rarefied conditions

Gas–solid flows occurring on very small spatial scales (of the order of micro and nanometres) are of great relevance in a number of industrial applications. It is currently not well established how particle motion and filtration are affected by non-isothermal conditions at such scales. Furthermore, when the particle size is comparable to the mean free path of the gas, rarefaction effects become important. In the present work we investigate the effects of heat transfer and non-isothermal conditions on the motion of small particles in rarefied flow. For that purpose, a suitable framework is developed here as a generic multiphase DNS method for rarefied flows. The resulting model is valid for low particle Reynolds number flows, irrespective of the Biot number, and for particle Knudsen numbers up to unity in unbounded flow. Using this model, we show that there is different settling behaviour of particles with an internal heat source in rarefied and continuum cases of the carrier gas respectively. It is shown that the chances for thermal levitation and/or lifting up of a particle due to buoyancy effects are significantly reduced under rarefied conditions.