Effect Of Entrance Region Research Articles

Hydrodynamic Analysis of Laminar Mixed Convective Flow of Ag-TiO2-Water Hybrid Nanofluid in a Horizontal Annulus

Nanofluids occupy a large place in many fields of technology due its improved heat transfer and pressure drop characteristics. Very recently, a new type of nanofluid, known as hybrid nanofluid, which consists of a mixture of two different nanoparticles suspended in the base fluid has been found to be the most emerging heat transfer fluid. It is well also established that entrance region effect enhances heat transfer rate. The present study deals with numerical investigations of the hydrodynamic behavior of the laminar mixed convective flow of a hybrid nanofluid in the entrance region of a horizontal annulus. A thermal boundary condition of uniform heat flux at the inner wall and an adiabatic outer wall is selected. The SIMPLER numerical algorithm is adopted in the present study. The hybrid nanofluid consists of water as base fluid and Ag-TiO2 as nanoparticles. The ratio of Ag to TiO2 is maintained as 1:3. The objective of the current study is mainly to analyze the hydrodynamic behavior hybrid nanofluid in the entrance region. The investigation reveals that the effect of the secondary flow due to the buoyancy forces is more intense in the upper part of the annular cross-section. It increases throughout the cross-section until its intensity reaches a maximum and then it becomes weak far downstream. The development of axial flow and temperature field is strongly influenced by the buoyancy forces.

Effect of variable fluid properties on the hydrodynamic and thermal characteristics of parallel flow microchannel heat exchanger

In this paper the parallel flow microchannel heat exchanger is numerically investigate and the effect of treating the properties of fluids as variables with temperature compared with constant fluid properties assumption on its performance is studied. A numerical simulation is made to solve 3D Navier stock equations in two fluids with temperature dependent properties and 3D energy equation in two fluids and solid walls taking into account the effect of entrance region and axial conduction in the walls. The properties entered to the main program by relations as a function of temperature using subroutines written to calculate the properties at the end of each iteration. The results obtained show that, treating the properties as a temperature dependent gives higher overall performance since it leads to obtain lower pressure drop and higher effectiveness compared with constant properties assumption.

Effect of operating conditions on the Cu2+ removal from wastewater by cementation on a fixed bed of zinc cylinders

This work investigates the effect of operating conditions on the rate of Cu2+ removal from synthetic waste of copper sulphate solution by cementation on a fixed bed of zinc cylinders under different conditions of solution flow rate, initial copper sulphate concentration, bed height, cylinder diameter and solution temperature. The values of the mass transfer coefficient was found to increase with increasing solution flow rate and temperature, while it decreased with increasing bed height, cylinders diameter and initial copper sulphate concentration. The value of the activation energy of the reaction ranged from 2.5 to 4.60 kCal/mol with lower and higher solution flow rates, respectively, which show that the reaction is diffusion controlled. The mass transfer correlation was deduced by dimensional analysis and was found to be in the form that: Sh = 2.1 Re0.867 Sc0.33 (d/h)−1.02. In addition, another correlation was deduced for the effect of entrance region by removing the calming section which was found to be in the form that: Sh = 2.30 Re0.870 Sc0.33 (d/h)−0.96. The obtained results can help in the design of a continuous flow reactor that may be used for the removal of heavy metals from wastewater.

Simultaneous development of flow and temperature fields in wire-wrapped fuel pin bundles of sodium cooled fast reactor

Simultaneous development of flow and temperature fields in the entrance region of fast breeder reactor (FBR) fuel pin bundles with helical spacer wires has been investigated by three-dimensional computational simulations. The Reynolds number, pitch of helical spacer wire and number of pins in the bundle are systematically varied. It is found that the magnitude of mean cross-stream velocity in the fully developed region is inversely proportional to the helical pitch length and it is nearly independent of the number of pins. But, there is a strong correlation between the locations of spacer wire and the peak cross-stream velocity. Flow attains full development at an axial length of 70 times hydraulic diameter in all the cases and this length is found to be unaffected by the helical pitch length. Friction factor is seen to fluctuate periodically over a mean value and the fluctuation over each helical pitch corresponds to a specific position of helical wire. The mean value of the friction factor in the entrance region reduces below the mean value in the fully developed region contrary to that seen in ducted flows. The mean fully developed friction factor is inversely proportional to the helical pitch. But, it is independent of the number of pins in the bundle. The Nusselt number passes through multiple minima before attaining fully developed periodic fluctuations and its development is slower than that of friction factor. For larger number of pins thermal development length is larger. Traditionally, the correlations reported for fully developed flow are considered for core design. But, the present study indicates that this approach is not conservative. Further, the entrance region effects and the oscillations in the fully developed region have to be properly accounted in the core design. Nusselt number exhibits a strong dependence on helical pitch similar to that of friction factor. A correlation for Nusselt number is proposed as a function of helical pitch and other influencing parameters. The hotspot factor in the pin bundle is found to vary from 3 to 4. Further, it increases, when either Reynolds number is low or the number of pins is more.

500W 급 마이크로 가스터빈을 위한 파형 휜을 가지는 리큐퍼레이터의 열성능 비교

In this study, thermal performance of recuperators with plain and offset strip fins is investigated to enhance the thermal efficiency of a micro gas turbine. Thermal cycle analysis is conducted to determine major design parameters of a single-pass counterflow recuperator. In order to evaluate the performance of the recuperator, the effectiveness and the pressure drop in the recuperators are chosen as the objective function and the design constraint, respectively. The optimized geometries for internal structure of the recuperators with plain and offset strip fins are obtained with varying the fin spacing and height. From the result, the recuperator with offset strip fins has better thermal performance when the fin spacing, s, is smaller than 1.45mm and the thermal performance of the recuperator with plain rectangular fins is higher than that with offset strip fins in the region of <TEX>$s{\geq}1.45mm$</TEX>. In addition, it is found that the entrance region effect and the longitudinal wall heat conduction effect should be taken into account for accurately predicting the thermal performance of the recuperators with both plain and offset strip fins.

Effect of entrance region and curvature on heat transfer in mini scale curved tubing at constant wall temperature

An experimental study on heat transfer enhancement in short curved tubes for the constant wall temperature boundary condition is presented. Copper tubing with four different radii of curvature (1cm, 2cm, 4cm, and 8cm) and in four lengths were used. Tubing radii were chosen, so that a constant tube length occurs in different curvatures. Hence, the effect of curvature on heat transfer at a constant length can be studied as well as the effect of heat transfer enhancement by changing the length in a constant curvature. Experimental results are also compared with an existing model for curved tubing. Water and three different silicone oils (0.65cSt, 1cSt and 3cSt) were used in the experiments to examine the effect of Prandtl number on curved tubing heat transfer augmentation.

Numerical and experimental performance analysis of rotary desiccant wheels

In air-conditioning field, a dehumidification has become increasingly important for human health and comfort especially in hot and humid climates. However, a conventional mechanical dehumidification with a vapor compression refrigerator has some problems. Therefore, much attention has been paid recently to a desiccant air-conditioning system as an alternative to the conventional system. In this paper, we focus on a rotary desiccant wheel which is the main component of the desiccant air-conditioning system and develop and validate the mathematical model by comparison with experimental results. The validation is conducted under various operating conditions. The mathematical model discussed in this paper includes, for example, the entrance region effect in air channel, detailed diffusion phenomenon in porous solid. In experiments, effects of the regeneration air temperature, air superficial velocity, wheel thickness and wheel rotational speed on the desiccant wheel performance are investigated. In addition, the temperature and humidity distribution at the outlet of the desiccant wheel are measured. As a result, an average relative error between the predicted and the measured humidity ratio difference distribution is 3.3% and temperature difference distribution 10.8%. Moreover, the effect of the regeneration air inlet temperature, the air superficial velocity, wheel thickness and wheel rotational speed on the desiccant wheel performance are clarified and the predicted results are totally in good agreement with the measured results.

Numerical Investigation on Heat Transfer and Fluid Flow Behaviors of Viscous Fluids in a Minichannel Heat Exchanger

Heat transfer and flow characteristics of different viscous liquids, such as engine oil, automatic transmission fluid, 50% ethylene glycol, and deionized water, were numerically analyzed through a multi-port serpentine minichannel heat exchanger (MICHX). The heat transfer performance of engine oil was found to be poor compared to other fluids. Pressure drop in MICHX for engine oil was found to be much greater compared to water. The distribution of temperature, mass flow rate, and heat transfer rate in each channel is more even for high-viscosity fluids compared to low-viscosity fluids. The entrance region effects on thermal and flow fields for high-viscosity fluids were observed negligible.

Effects of entrance region transport processes on free convection slip flow in vertical microchannels with isothermally heated walls

Natural convection gaseous slip flows in vertical microchannels with isothermal wall conditions are numerically investigated, in order to analyze the influence of the entrance (developing) region on the overall heat transfer characteristics. A long channel aspect ratio is considered, so as to achieve both hydrodynamically and thermally fully developed conditions at the channel exit. In other words, the flow-field within the microchannel consists of both developing and fully developed regimes. A wide range of Rayleigh number is covered, so that the cases of very short and relatively large entrance lengths can be analyzed in the same unified mathematical framework. With first order velocity and temperature jump conditions at the microchannel walls, local and average Nusselt number values are computed, by invoking the Navier Stokes equation and the energy conservation equation. It is recognized that the micro-scale effects, being associated with the velocity slip and temperature jump conditions, exhibit enhancements in the rate of heat transfer, as compared to the similar macro-scale geometries. The relative enhancements in the average Nusselt number become more prominent for higher values of Knudsen number, whereas this augmentation effect is found to be somewhat arrested at higher values of Rayleigh number. Contrasting features in the heat transfer rate predictions with and without the considerations of the entrance region effects are also carefully noted.

Effectiveness of a Spiral-Plate Heat Exchanger With Equal Capacitance Rates

Abstract A formula is derived for the dependence of heat exchanger effectiveness on the number of transfer units for a spiral-plate heat exchanger with equal capacitance rates. The difference-differential equations that describe the temperature distributions of the two counter-flowing fluids, neglecting the effects of thermal radiation, are solved symbolically to close approximation. Provision is made for the offset inlet and exit of the hot and cold fluids at the outer periphery and for large heat transfer coefficients in the entrance regions. The peak effectiveness and the number of transfer units at which it occurs are linear functions of the maximum angle of the Archimedean spiral that describes the ducts; entrance region effects reduce both.

Mixed convection heat transfer in the entrance region of horizontal annuli

The present study deals with a numerical investigation of steady laminar mixed convection heat transfer in a horizontal concentric annulus using air and water as the working fluid. Special attention has been paid to the entrance region effect. The thermal boundary condition chosen is that of uniform heat flux at the inner wall and an adiabatic outer wall. The basic numerical procedure used is the SIMPLE algorithm [Int. J. Heat Mass Transfer 15 (1972) 1787] which uses the finite difference method for solving the momentum and energy equations. The investigations reveal that the Nusselt numbers for air are considerably greater than the corresponding values for fully developed mixed convection values over a significant portion of the annular duct. The numerical results were supplemented with some experimentally obtained data.

NUMERICAL STUDY OF STEADY LAMINAR FLOW THROUGH TUBES WITH MULTIPLE CONSTRICTIONS USING CURVILINEAR CO-ORDINATES

The flow field through tubes with multiple axisymmetric constrictions in tubes was studied numerically. Two practical problem cases were considered and the numerical scheme was developed for both. In the first case there are one, two, three and four constrictions in the tube. The effects of the number of constrictions on wall shear stress, pressure drop, streamline, vorticity and velocity distributions as the flow passes through the tube were studied and the development of the periodicity characteristics was investigated. In the second case there were multiple constrictions in the tube equidistant from each other. For this case the governing equations were reformulated for a module at a sufficient distance downstream from the inlet where the entrance region effects could be ignored and flow field is assumed to repeat itself. The flow field solutions were obtained in this region. The governing equations were formulated in curvilinear co-ordinates and a finite volume discretization procedure was used to solve the problem. The computations were carried out over a range of Reynolds numbers between 50 to 250 for constrictions with 75 percent area reduction. The method is validated by comparing some of the solutions with experimental results.

Laminar start‐up flow in short pipe lengths

AbstractA solution method is described for the start‐up flow of an incompressible Newtonian fluid in a short pipe. The complete Navier‐Stokes equations are solved numerically using a fragmented control volume method. The solution procedure involves a novel approach for accommodating the time‐dependent condition for pressure at the pipe inlet. It is shown that the start‐up time for short pipes is less than that for long pipes, due to kinetic head development and entrance region effects. Correlations are presented for estimating the start‐up time.

Effect of Entrance Region on Laminar Startup Flow in Pipes

The ultimate effects of the entrance region on startup flow are considered by one-dimensional steady-state analysis. By roughly accounting for the entrance loss, semiempirical formulas for the average velocity and the hydrodynamic entrance length in terms of a startup flow parameter and an entrance effect coefficient are provided. The startup flow parameter M = gHD4/2048v2L2 is identified as the fundamental parameter of the problem. While Szymanski’s solution is recovered in the long pipe limit, i.e., M → 0, the steady-state volume flux through short pipes (M &gt; 0.05) is significantly reduced due to entrance region effects.

An analysis on entrance region effect of the laminar radial flow between two parallel disks

In this paper, B. B. Golubef method[1] is used for calculating the radial diffuse flow between two parallel disks for the first step. The momentum integral equation together with the energy integral equation is derived from the boundary layer momentum equation, and the expression of secondary approximation explicit function in which the channel length of entrance region varies with the boundary layer thickness can be obtained by using Picard iteration[2] in the solution of the energy integral equation. Therefore, this has made it possible to analyze directly and analytically the coefficients of the entrance region effect. In particular, when the outer diameter of disk is smaller than the entrance region length, the advantage of this method can be prominently manifest.

Study on correction coefficients of liminar and turbulent entrance region effect in round pipe

In this paper, a universal method for calculation of the inlet length and correction coefficient for the pressure losses and the flow in the case of laminar and turbulent flow in a smooth round pipe is put forward.

A two-dimensional model of second order rapid reactions in turbulent tubular reactors

Previous studies of second order rapid reactions in turbulent tubular reactors have been analyzed in terms of a one-dimensional model. Deviations between theory and experiment have been interpreted in terms of diffusion limited reaction rates. In this paper the experimental results are re-analyzed in terms of a two-dimensional model leading to the conclusion that mixing and entrance region effects are responsible for the deviations between the experimental results and one-dimensional theory. A new upper bound for the Damköhler number associated with diffusion controlled reaction rates is suggested.

Turbulent heat and momentum transfer for gases in a circular tube at wall to bulk temperature ratios to seven

Measurements are reported for local friction and heat-transfer coefficients with constant heat flux in an electrically heated circular tube. Precooled nitrogen in turbulent flow was used to obtain local wall to bulk temperature ratios from 1·24 to 7·54. The Inconel test section had a 40 diameter hydrodynamic entry section and a 160 diameter heated section. The bulk inlet Reynolds numbers covered the range from 18 300 to 279 000. Correlations in terms of both the bulk and the modified wall Reynolds numbers are presented for the local friction factors; the local heat-transfer results are presented in terms of the wall to bulk temperature ratio with bulk properties and also directly on the modified wall Reynolds number. Both the friction and heat-transfer results are shown to be significantly affected by the influence of the variable properties. Entrance region effects for both the heat-transfer and friction coefficients are discussed.