O. Zeitoun

Natural convection in a horizontal cylindrical annulus using porous fins

Purpose – The aim is to study the effects of fin conductivity ratio, Darcy number, and Rayleigh number on the average Nusselt number for fins made of porous material when attached to the inner cylinder of the annulus between two concentric cylinders. The paper also aims to compare the results with those obtained using solid fins over a range of Rayleigh numbers.Design/methodology/approach – The Darcy‐Brinkman equations were used to model the fluid flow inside the porous media and the Boussinesq approximation was used to model the buoyancy effect. The energy equation is also solved to find the temperature distribution in the domain of interest. The model equations are solved numerically using a finite volume code.Findings – Porous fins provided higher heat transfer rates than solid fins for similar configurations. This enhancement in heat transfer reached 75 per cent at Ra=5 × 104 and Da=2.5 × 10−2. It is also found that unlike solid fins the rate of heat transfer from the cylinder equipped with porous fin...

Interfacial Heat Transfer Between Steam Bubbles and Subcooled Water in Vertical Upward Flow

In two-fluid modeling, accurate prediction of the interfacial transport of mass, momentum, and energy is required. Experiments were carried out to obtain a data base for the development of interfacial transport models, or correlations, for subcooled water-steam bubbly flow in vertical conduits. The experimental data of interest included the interfacial area concentration, interfacial condensation heat transfer, and bubble relative velocity. In the present investigation, bubble condensation in subcooled water-steam flow in a vertical annulus at low flow rate and low pressure is investigated experimentally. A high-speed video system (up to 1000 frame/s) was used to visualize two orthogonal views of the flow simultaneously. A digital image processing technique was used to track and measure the velocity and size of the collapsing bubbles. The axial void fraction distribution was also measured by a single beam gamma densitometer.

Measurement of interfacial area concentration in subcooled liquid-vapour flow

Abstract In two-fluid modelling, accurate prediction of the interfacial transport of mass, momentum and energy is required. Experiments were carried out to obtain a database for the development of interfacial transport models, or correlations, for subcooled water-steam flow in vertical conduits. The experimental data of interest included the interfacial area concentration, interfacial condensation heat transfer and bubble relative velocity. This paper focuses on the interfacial area concentration. The interfacial area concentration was obtained by measuring the distributions of bubble volume and surface area as well as the area-averaged void fraction at various axial locations in subcooled water-steam condensing vertical upward flow under low flow rate and low pressure conditions. The bubble size and surface area were determined using high-speed photography and digital image processing techniques. The area-averaged void fraction was measured by a single-beam gamma densitometer. The results were compared with existing correlations, which were developed on the basis of data obtained for air-water adiabatic flows. Poor agreement between the present data and the existing correlations was obtained. Accordingly, new correlations suitable for subcooled liquid-vapour bubbly flow are proposed.

Nanofluid impingement jet heat transfer

Experimental investigation to study the heat transfer between a vertical round alumina-water nanofluid jet and a horizontal circular round surface is carried out. Different jet flow rates, jet nozzle diameters, various circular disk diameters and three nanoparticles concentrations (0, 6.6 and 10%, respectively) are used. The experimental results indicate that using nanofluid as a heat transfer carrier can enhance the heat transfer process. For the same Reynolds number, the experimental data show an increase in the Nusselt numbers as the nanoparticle concentration increases. Size of heating disk diameters shows reverse effect on heat transfer. It is also found that presenting the data in terms of Reynolds number at impingement jet diameter can take into account on both effects of jet heights and nozzle diameter. Presenting the data in terms of Peclet numbers, at fixed impingement nozzle diameter, makes the data less sensitive to the percentage change of the nanoparticle concentrations. Finally, general heat transfer correlation is obtained verses Peclet numbers using nanoparticle concentrations and the nozzle diameter ratio as parameters.

NUMERICAL INVESTIGATION OF NATURAL CONVECTION AROUND ISOTHERMAL HORIZONTAL RECTANGULAR DUCTS

Two-dimensional laminar natural-convection heat transfer in air around horizontal ducts with rectangular and square cross sections is studied numerically. Different aspect ratios are used for wide ranges of Rayleigh numbers. Results are presented in the form of streamlines and isothermal plots around the circumference of the ducts. The computational procedure is based on the finite-element technique. Temperature and velocity profiles are obtained near each surface of the ducts. Reverse flow and circulations are observed at high aspect ratios. Heat transfer data are generated and presented in terms of Nusselt number versus Rayleigh number for different aspect ratios. Correlation covering the aspect ratios used is obtained in dimensionless form of Nusselt number, Rayleigh number, and aspect ratio.

The Effect of Alumina–Water Nanofluid on Natural Convection Heat Transfer Inside Vertical Circular Enclosures Heated from Above

Experimental investigation on natural convection heat transfer is carried out inside vertical circular enclosures filled with Al2O3–water nanofluid with different concentrations; 0.0%, 0.85% (0.21%), 1.98 (0.51%), and 2.95% (0.75%) by mass (volume). Two enclosures are used with 0.20 m inside diameter and with two different aspect ratios. The top surface of the enclosure is heated using a constant-heat-flux flexible foil heater while the bottom surface is subject to cooling using an ambient air stream. Various heat fluxes are used to generate heat transfer through the nanofluid. The average Nusselt number is obtained for each enclosure and correlated with the modified Rayleigh number using the concentration ratio as a parameter. A general correlation for the average Nusselt number with the modified Rayleigh number is obtained using the volume fraction and the aspect ratio as parameters to cover both enclosures. The results show that the Nusselt number for the alumina–water nanofluid is less than that of the base fluid. This means that using the alumina–water nanofluids adversely affects the heat transfer coefficient compared to using pure water. It is also found that the degree of deterioration depends on the concentration ratio as well as the aspect ratio of the enclosure.

Nanofluids forced convection heat transfer inside circular tubes

Two-dimensional turbulent convective heat transfer behaviour of alumina (Al2O3) nanoparticle dispersion in water flow in a horizontal circular pipe at constant wall temperature is investigated numerically. The computational procedure is based on the finite-volume technique. Three stream velocities corresponding to 5,000 < Re < 4 × 106 and five different concentrations of nanoparticle (0%, 1%, 2%, 4% and 6%) are studied. The full range of flow at the entrance length and the fully developed are considered. The shear stress are observed to increase at any x station along the pipe as the concentration of nanoparticle increase and it attains its higher value at the beginning of the pipe at the entrance region and then drops to an asymptotic value at the fully developed region. However, the case is reversed for Nusselt numbers along the pipe wall where they decrease as the concentration increase at each specific velocity value and in general as the velocity increases both Nusselt number and the shear stress increase. Different envelopes are obtained for Nusselt numbers and the shear stress in terms of Reynolds number. Finally, Reynolds number is observed to decrease as the concentration increase at fixed inlet velocity.

Numerical Modeling of Natural Convection Heat Transfer around Horizontal Triangular Cylinders

Laminar natural convection heat transfer from uniformly heated horizontal cylinders of a triangular cross-section is investigated numerically in air. Different cylinder cross-section dimensions and orientations are examined under laminar conditions. The computational procedure is based on the finite volume technique (using the commercial software FLOWORKS). Results are presented in the form of streamline and temperature contour plots around the perimeter of the cylinders. Some representations of thermal layers around the side of the cylinders are obtained for different modified Rayleigh numbers. General correlations of Nusselt numbers verse the modified Rayleigh numbers are obtained. Symmetric, transition, and asymmetric plumes are obtained and characterized for both positions of the cylinders.

Heat Transfer Between a Vertical Water Jet and a Horizontal Square Surface

Flow visualization was used to measure hydraulic jump radius on a square plate. Heat transfer between a water jet and a horizontal square surface was investigated experimentally for different jets and square sizes. Effects of flow rates on heat transfer were investigated. The data are represented in terms of Nusselt number as a function of Reynolds and Peclet numbers. A correlation for the Nusselt number in terms of the Peclet number and L/Di was obtained. The proposed correlation predicts the current data of heat transfer very well.

Nanofluids natural convection heat transfer in horizontal annulus

The effect of using nanoparticles on natural convection heat transfer in horizontal cylindrical annuli was investigated numerically using finite volume technique together with SIMPLE algorithm for Do/Di = 2 and 1.25. Aluminium oxide (Al2O3) nanoparticles in water as base fluid were used in the current investigation. Laminar conditions up to Rayleigh number Ra of 109 were investigated for 0%, 1% and 4% nanoparticle concentrations. The results indicated that nanoparticles can increase heat transfer in the conduction dominated regime while it decreases it in the convection dominated regime.