Zalman Lavan

Experimental study of thermally stratified hot water storage tanks

Abstract Temperature stratification in hot water storage systems was studied experimentally. In particular, high extraction rates from plastic cylindrical vessels were emphasized. Data were taken at various length to diameter ratios, inlet-outlet temperature differences and mass flow rates. The effect of inlet and exit port configuration on thermal stratification was also studied. The data were empirically correlated to yield useful relations for the design of effective hot water storage systems. Finally, a novel inlet and exit configuration scheme was designed for an 80 gal. (300 l.) and a 500 gal. (1900 l.) storage tank.

Starting vortex, separation bubbles and stall: a numerical study of laminar unsteady flow around an airfoil

Abstract : The stalling characteristics of an airfoil in laminar viscous incompressible fluid are investigated. The governing equations in terms of vorticity and stream function are solved utilizing an implicit finite difference scheme and point successive relaxation procedure. The development of the impulsively started flow, the initial generation of circulation and the behavior of the forces at large time are studied with emphasis on the formation region. Following incipient separation, the lift increases due to enlargement of a separation bubble and intensification of the flow rotation in it. The extension of this bubble and beyond the trailing edge causes its rupture and brings about the stalling characteristics of the airfoil. The lift increases when attached clockwise bubbles grow and anti-clockwise bubbles are swept away and vice versa. (Author Modified Abstract)

Entropy Generation in Convective Heat Transfer and Isothermal Convective Mass Transfer

The irreversible generation of entropy for two limiting cases of combined forced-convection heat and mass transfer in a two-dimensional channel are investigated. First, convective heat transfer in a channel with either constant heat flux or constant surface temperature boundary conditions are considered for laminar and turbulent flow. The entropy generation is minimized to yield expressions for optimum plate spacing and optimum Reynolds numbers for both boundary conditions and flow rigimes. Second, isothermal convective mass transfer in a channel is considered, assuming the diffusing substance to be an ideal gas with Lewis number equal to unity. The flow is considered to be either laminar or turbulent with boundary conditions at the channel walls of either constant concentration or constant mass flux. The analogy between heat and mass transfer is used to determine the entropy generation and the relations for optimum plate spacing and Reynolds number. The applicable range of the results for both limiting cases are then investigated by non-dimensionalizing the entropy generation equation.

Optimization of wet-surface heat exchangers

Wet-surface heat exchangers are analyzed in order to determine the configuration that optimizes the performance. The objective is to cool a stream of air to a temperature lower than the inlet wet-bulb temperature by the evaporation of water. Three laboratory models and a commercial prototype were analyzed. They are a unidirectional, a counter-flow, a counter-flow closed-loop configuration and a cross-flow closed-loop commercial unit, respectively. It was found that dry-bulb temperatures considerably lower than the inlet wet-bulb temperature can be easily achieved. In fact, the inlet dew-point temperature can be approached with moderate flow rates and simple geometries.

Entropy generation in combined heat and mass transfer

Abstract Irreversible entropy generation for combined forced convection heat and mass transfer in a twodimensional channel is investigated. The heat and mass transfer rates are assumed to be constant on both channel walls. For the case of laminar flow, the entropy generation is obtained as a function of velocity, temperature, concentration gradients and the physical properties of the fluid. The analogy between heat and mass transfer is used to obtain the concentration profile for the diffusing species. The optimum plate spacing is determined, considering that either the mass flow rate or the channel length are fixed. For the turbulent flow regime, a control volume approach that uses heat and mass transfer correlations is developed to obtain the entropy generation and optimum plate spacing.

Separation and Flow Reversal in Swirling Flows in Circular Ducts

The swirling motion of a laminar incompressible viscous flow in a circular duct is studied. The duct consists of two smoothly joined sections, one stationary and the other rotating with a constant angular velocity. A linearized analytical solution, valid for flows having small Reynolds numbers and large swirl ratios, is developed. Solutions for a wider range of Reynolds numbers and swirl ratios are obtained by numerically solving the discretized angular momentum and vorticity transport equations. The occurrence of flow reversal on the axis and near the tube wall is studied, in particular, and conditions for incipient flow reversal are established.

Flow in a Two-Dimensional Channel With a Rectangular Cavity

Flow structure in rectangular cavity in lower wall of two dimensional channel for various aspect ratios and Reynolds numbers

Stability of Pipe Flow with Superposed Solid Body Rotation

The stability of a flow consisting of solid body rotation superposed on an entrance‐type pipe flow profile is investigated experimentally. Increasing the swirl ratios from zero to four reduced the axial Reynolds number at which laminar flow breaks down from 2500 to 900.

Second-law analysis of a two-dimensional regenerator

A second-law analysis of a two-dimensional, fixed-bed regenerator is presented. We assume that the solid matrix has infinite thermal resistance in the flow direction and finite thermal resistance, characterized by the matrix Biot number, perpendicular to the flow direction. A trade-off between mechanical and thermal exergy losses yields optimal Ntu, CminCr and effectiveness. Therefore, by specifying a channel geometry, mass flow-rate, matrix porosity and total frontal area, the optimum channel length and operating period can be determined. Increasing the matrix Biot number causes a reduction in the second-law efficiency at high effectiveness and a reduction in the optimum effectiveness corresponding to the maximum second-law efficiency.

Thermal performance of a serpentine absorber plate

The heat removal factor of a flat absorber plate with a serpentine tube, making N 180/sup 0/ turns, is investigated analytically. Results are obtained for N=1,2,3 and 4 by solving a set of matric differential equations. The results are presented in terms of generalized nondimensional parameters.