Convective Transfer Rate Research Articles

Numerical Study of Heat Transfer in a Partitioned Cavity Containing a Porous Medium

Cavities separated by multiple vertical partitions and filled with a porous medium present a remarkable thermal insulation quality, offering potential solutions in various engineering fields. The aim of this study is to analyze the impact of the presence of a porous medium on heat transfer through a partitioned cavity. We have developed a numerical model based on the Navier-Stokes and heat transfer equations, solved using Ansys Fluent software. We examine the evolution of the Nusselt number (convection and radiation) as a function of the position of the porous medium inside the cavity, as well as physical properties such as emissivity, wall conductivity and Rayleigh number. Current lines and isotherms are obtained from this numerical model. Nusselt numbers for both convection and radiation are calculated, taking into account the position of the porous medium in the system, as well as the effect of varying physical parameters on heat transfer. It has been observed that the presence of the porous medium leads to a reduction in the rate of heat transfer within the cavity. The further the porous medium is from the hot wall, the more pronounced this reduction. In addition, radiative transfer has a downward influence on convective transfer. Furthermore, the convective transfer rate decreases with increasing emissivity. As far as conductivity is concerned, transfer rates (convective and radiative) initially increase until a maximum Nusselt number is reached, after which they gradually decrease with a further increase in conductivity. Nusselt numbers (convection and radiation) increase as the Rayleigh number increases.

Analysis of Heat Transfer and Efficiency of Rack Type Flash Dryer Machine in Purple Sweet Potato Drying Process

Purple sweet potato (Ipomoea batatas var Ayumurasaki) has deep purple flesh and tuber skin. The variety of processed purple sweet potato products is increasingly diverse as purple sweet potato production increases, such as cookies and chips. Therefore, a process is needed to turn purple sweet potato into flour before being made into a variety of processed products, namely the drying process. Drying is a process of reducing the water content of a material to a certain level. To determine how much fuel is used by dryer machine, it is necessary to evaluate the effectiveness of the machine. So that in this study the aims were to determine the amount of energy needed to dry purple sweet potatoes, calculate the efficiency of the flash dryer machine and find out the temperature distribution on each tray of the drying machine. Based on the research results of the rack-type flash dryer drying machine, it can be concluded that the mechanism of the heat transfer rate in the purple sweet potato drying process occurs by forced convection heat transfer from the drying air to the purple sweet potatoes and a mass flow rate value of 0.1272 kg/s is obtained, the convection transfer rate in the drying chamber is 836.84 W and the sensible heat value is 1033.84 W. Hence, the amount of heat to dry 3 kg of purple sweet potato for 4 hours is 1870.68 W and an efficiency value of 55.26% is obtained. The total energy required to dry 3 kg of purple sweet potato in 4 hours is 5712.86 kJ.

МЕТОДИЧЕСКИЕ ПОЛОЖЕНИЯ КОМПЛЕКСНОЙ ЭКОЛОГИЧЕСКОЙ ОЦЕНКИ ВОЗДЕЙСТВИЯ ПОРОДНЫХ ОТВАЛОВ ШАХТ НА ОКРУЖАЮЩУЮ СРЕДУ

It is shown that throughout all geo-technological periods of the existence of coal mines, there is a negative impact of waste dumps on the atmosphere, water resources and soil. Theoretical propositions have been formulated, in accordance with which the migration of liquid pollutants into the soil laver and underlving rocks in the zone of action of the waste dump is described by a one-dimensional equation of convective diffusion, taking into account the kinetics of sorption of pollutants. It is recommended to model the air movement when flowing around the waste dumps by the O. Reynolds system of equations for the turbulent motion of a viscous fluid, using the finite volume methodfor the numerical solution. The numerical solution of the equations of motion of O. Reynolds makes it possible to visualize the picture of air flow in special zones and predict the dust and gas transport in the surface layer of the atmosphere. It is expedient to simulate the dust and gas transfer by the equation of convective-turbulent diffusion, setting the convective transfer rate according to the results of calculating the velocity fieldformed when flowing around the dump.

Maximum Density Effects on Natural Convection over an Isothermal Vertical Plate Embedded in an Anisotropic Porous Medium

Keywords: Natural convection, anisotropic porous medium, inversion of density.Abstract. The natural convection in the porous medium has attracted considerable attention with its applications in various industrial sectors, such as the agro-alimentary, the pharmaceutical and oil processing industries. The present work is about the study of the phenomenon of natural convection at 4°C on the vertical plate lining in an anisotropic porous medium in permeability. The wall of the vertical surface is subjected to a constant temperature with defined hydrodynamic conditions and thermal limits. Generalized Darcy’s law was used to establish the governing equations of the system. The control parameters governing the system are respectively the permeability ratio K*, the angle ϴ of the major axis and the inversion parameter R. The general basic equations were solved numerically using Runge-Kutta and shooting method. There was validated against previous work. The effect of these parameters on the heat transfer was highlighted. From the analysis result, it comes out the following conclusions: The convective flow is significantly affected by the anisotropic parameter; heat transfer along the vertical surface is maximum (minimum) when the main shaft having the high permeability is oriented parallel (perpendicular) to the gravitational field; the rate of heat transfer is depending on the inversion parameter R. The convective transfer rate illustrated by the local Nusselt number is symmetric with respect to R=0.45, where it reaches its smallest value. It is inversely proportional to the distribution of the thickness of boundary layer. A high convective transfer rate corresponds to a low boundary layer thickness and this inversely.

Heterogeneous convective thermal and airborne pollutant removals from a partial building enclosure with a conducting baffle: Parametric investigations and steady transition flow solutions

Natural convection on a heated and polluted strip within a partial enclosure is investigated, where the baffle is attached to the right wall. Numerical solutions demonstrate that the baffle in the partial enclosure could enhance heat and mass transfer rates only when Ra (thermal Rayleigh number) is higher than the critical value and its dimensionless vertical deviation between the baffle and horizontal x-axis, represented by E, lies between −0.8 and −0.5. When Ra≥105 and E=−0.50 are imposed, heat and mass transfer rates for the dimensionless length of the baffle L=0.8 are higher than those of L=0.2 and L=0.5; whereas, for Ra ≥8.0×105 and E=−0.8, convective transfer rates of L=0.5 and L=0.8 are higher than those of L=0.2. These trends could be reversed when the regime of low Ra is established. Steady transition flows are observed as Ra varies from 1.25×105 towards 1.30×105, where L=0.2 and E=−0.6 are maintained; As Ra turns up to 8.00×105 from 7.90×105, Nu and Sh could be enhanced sharply for L=0.8 and E=−0.7. Moreover, huge divergences are observed for temperature and concentration distributions along the central axis when the steady transition flow appears in the enclosure. Present work could be significant for natural convection optimization and passive control of heat and pollutant removals from the building enclosures or electronic boxes.

Annular Impinging Jet Controlled by Radial Synthetic Jets

This experimental study focuses on generation and control of annular impinging jets. The annular nozzle used in the investigations was designed with an active flow control system using 12 synthetic jets issuing radially from the central nozzle body. Measurements of the control effects were made on the impingement wall. The data acquisition involved wall pressure and wall mass transfer (by the naphthalene sublimation technique) using air as the working fluid. Also measured was time-mean flow velocity (by a Pitot probe) in the jet flow field. Moreover, flow visualization was carried out. Two main flow-field patterns (A and B) were identified. The patterns differ in the size of the separated-flow recirculation regions that develop attached to the nozzle central body: While pattern A is characterized by a quite small recirculation region (bubble) extending not far from the nozzle exit, pattern B exhibits a large recirculation region, reaching up to the impingement wall, on which it forms a stagnation circle. The control action modifies the flow field, resulting in changes of the corresponding heat/mass transfer distributions. The convective transfer rate on the stagnation circle can be demonstrably enhanced by 20% at a moderate nozzle-to-wall distance, equal to 0.6 of the nozzle outer diameter.

The recycle effect on the collector efficiency improvement of double-pass sheet-and-tube solar water heaters with external recycle

The paper presents a theoretical prediction of the performance of a double-pass sheet-and-tube solar water heater with external recycle and comparison with that of a conventional type collector. Analytical results show that the recycle effect can effectively enhance the collector efficiency compared with that in a single-pass device with the same flow rate. The desirable effect of increasing convective transfer rate and the undesirable effect of reducing heat-transfer driving force are the two conflict effects produced by the recycle operation. It was found that the increment of convective transfer rate by increasing the recycle ratio could generally compensate for the decrement of the temperature difference, leading to improved performance, especially for low flow rate. Also, the collector efficiency increases with increasing collector aspect ratio (i.e. either increasing the tube length or decreasing the number of tube pairs) for a specified collector area and the distance between tubes.

Thermal transport due to material and gas flow in a furnace for drawing an optical fiber

The transport processes involved in the neck-down region for optical fiber drawing are numerically investigated. In this manufacturing process, a moving glass rod is heated in a furnace containing an inert gas environment and drawn into a thin optical fiber. The conjugate problem is solved considering both radiation and convection, with focus on the latter. Two different flow configurations, involving inert gas flow in the same as well as in the opposite direction as the moving preform/fiber, are considered in this study. A coordinate transformation is used to change the complicated computational domains in the gas and the fiber to cylindrical ones. The transport in the fiber is coupled with that in the gas through the boundary conditions. The radiative thermal transport is calculated using an enclosure model developed in an earlier study. The numerical results on convective flow and transport are validated by comparing with results available in the literaturefor simpler configurations. The effects of several important parameters such as fiber draw speed, inert gas velocity, furnace dimensions, and gas properties on the flow and temperature distributions are investigated. For the aiding flow case, in which the inert gases flow in the same direction as the fiber, heat transfer to the fiber increases as the gas velocity increases. For opposing flow, a recirculating region appears in the gas, close to the moving fiber surface, causing reduction in heat transfer as compared to the aiding case. The thickness of this recirculating zone decreases with increasing inert gas velocity. Radiation is found to be the dominant mode of heat transfer in the overall heating of the preform/fiber, with nitrogen as the inert gas. However, near the edges of the furnace, radiation heat transfer is relatively small and convection becomes very important. Also, the convective transfer rate is relatively large near the flow entrance because of the large temperature difference between the gas and the fiber. However, away from the entrance, the gas heats up and the temperature difference relative to the fiber decreases, resulting in a smaller convective heat transfer rate. The relevance of the results to various aspects of the fiber-drawing process is discussed.

Pulse Combustion: Tailpipe Exit Jet Characteristics*

Abstract A preliminary study of the convective transfer rates of scalar quantities in a strongly oscillating free jet (+70 m/s to -50 m/s at 100 Hz generated by a pulse combustor) has been performed. This work is described in two parts: This paper discusses the free jet character, whereas the companion paper discusses the impinging jet characteristics associated with the convective heat transfer rate to a plate normal to the jet axis. In this paper the essential differences between the steady and oscillating jets are described. Rayleigh temperature measurements and flow visualizations (schlieren photography and laser Mie scattering) were used to quantify the jet characteristics. The method of vortex dynamics was used to model the vorticity and velocity fields of the pulsating jet. The modeling results support the conclusions drawn from experimental data. The steady jet exhibited typical axial and radial temperature profiles for an axisymmetric jet and, hence, it had typical decay rates. The character of the oscillating jet was dominated by the formation of toroidal vortex elements during the positive velocity portion of the cycle. The entrainment of ambient fluid and, hence, the mixing of this fluid with that of the jet was an order of magnitude greater for the oscillating jet than for the steady jet. Thus, the temperature decay rate was also an order of magnitude higher than that of the steady jet. The high peak velocities, finer scale of turbulence, and the coherent toroidal vortex of the oscillating jet are believed to be responsible for the improved scalar transport.

Variational analysis of high mass transfer rates from spherical particles boundary-layer injection suction considerations at low particle reynolds numbers and high peclet numbers

A general variational analysis for hydrodynamic, thermal, and diffusional boundary-layer flows previously developed by the authors is adapted here to characterise relatively high mass transfer rates from moving spherical drops, bubbles, or solid particles. This work takes into account the effect of radial velocity at the particle surface (due to the very process of interfacial mass transfer) on the analysis of convective mass transfer rates-an important effect which has been neglected so far in studies of droplet dynamics. The error introduced by neglecting this effect is shown to amount to tens of per cents even when the interfacial radial velocity due to mass transfer is relatively small. In general it is found that the errors introduced in predicting convective transfer rates or boundary-layer thickness with neglected radial velocity at the interface are much more pronounced when circulation inside fluid particles is small. Comparison of our method with existing “exact” or integral methods (for calculating the mass transfer coefficients with a neglected radial velocity) clearly demonstrates its general property to extract the “best” trial solution from any assumed family of possible approximate solutions.

Experimental investigation of heat transfer at an air‐Earth interface

This paper is concerned with the instrumentation for, and the results from, an experimental investigation of heat transfer over a smooth dry‐lake. The original data are on file in the Department of Engineering, University of California, Los Angeles. Heat‐transfer rates into the ground were measured with thermopile heat meters; the net radiant exchange at the Earth's surface was measured with a net exchange radiometer. A continuous 48‐hour record of these quantities, together with vertical convective heat‐transfer rates determined from heat balances at the surface, is presented. Unit convective conductances (heat‐transfer coefficient's) are calculated from the convective transfer rates and differences between the surface temperature and the air temperature at a height of two meters. An empirical expression for the convective conductance, which agrees within ten per cent with the experimentally determined values, is presented.A series of small thermocouples was used to measure the air‐temperature distribution very close to the surface. Vertical convective heat‐transfer rates calculated from these measured temperature gradients compare favorably with those from the heat balances. These measurements indicate the probable existence of a laminar sublayer, though further data are necessary for conclusive affirmation.