Ahmad Fakheri

A Hybrid Approach for Full Numerical Simulation of Heat Exchangers

Understanding the details of flow and temperature fields is important in improving the performance of the heat exchangers. The full numerical solution of heat exchangers are computationally prohibitive because the flow and temperature fields must be simultaneously determined in at least two fluids and the solid surfaces separating the two fluids, making full simulation of heat exchangers practically nonexistent. As complicated as the flow is, there are certain characteristics of heat exchangers that if taken advantage of, make the numerical simulation of the heat exchangers more practical. In this paper, the idea of a hybrid approach is proposed, in which the nonlinear momentum equations for one or two channels are solved by using commercially available CFD codes. The velocity field is then inputted into a user-developed code for solving the linear energy equation for the entire heat exchanger. This is an efficient approach which has been successfully used to fully simulate a parallel flow heat exchanger.Copyright

Heat Transfer in Pipe Flow

A simple procedure using spreadsheets is presented where the temperature distribution for laminar flow in a circular pipe is determined from the entrance of the pipe up to the fully developed region is calculated numerically. The results are then used to show the different features of internal flow like constancy of Nusselt number. The solution is presented for both isothermal and uniform heat flux boundary conditions and are then compared with available correlations.Copyright

On Application of the Second Law to Heat Exchangers

The application of entropy minimization to heat exchangers leads to inconsistent results and does not yield much useful design information. In this paper it is shown that in applying the second law to heat exchangers, three assumptions are typically made that are incorrect and that once they are removed, useful and consistent results are obtained from the second law. In addition, a new performance measure, entropy flux is introduced and it is shown that the objective in heat exchanger design should be the maximization of the entropy flux.Copyright

Thermal Efficiency of the Cross Flow Heat Exchangers

The heat exchanger efficiency is defined as the ratio of the actual heat transfer in a heat exchanger to the optimum heat transfer rate. The optimum heat transfer rate, qopt , is given by the product of UA and the Arithmetic Mean Temperature Difference, which is the difference between the average temperatures of hot and cold fluids. The actual rate of heat transfer in a heat exchanger is always less than this optimum value, which takes place in an ideal balanced counter flow heat exchanger. It has been shown that for parallel flow, counter flow, and shell and tube heat exchanger the efficiency is only a function of a single nondimensional parameter called Fin Analogy Number. The function defining the efficiency of these heat exchangers is identical to that of a constant area fin with an insulated tip. This paper presents exact expressions for the efficiencies of the different cross flow heat exchangers. It is shown that by generalizing the definition of Fa, very accurate results can be obtained by using the same algebraic expression, or a single algebraic expression can be used to assess the performance of a variety of commonly used heat exchangers.Copyright

Transient Analysis of Heterogeneous and Homogeneous Combustion in Boundary Layer Flow

Abstract Abstract-The present effort is an analytical study of combined homogeneous and catalytic combustion. Numerical solutions are obtained for the transient combustion of a mixture of fuel and oxidizer in boundary layer flow. The effects of an isothermal as well as an adiabatic catalyst on homogeneous combustion are examined. For an isothermal plate, the addition of the catalyst delays the ignition process, increases the ignition distance and lowers the gas phase temperatures. However, shorter concentration conversion distances and more uniform plate energy flux distributions are obtained. In general, increasing the plate temperature lowers the ignition distance while the influence of chemical parameters depends on the relative importance of each mode of combustion. The effects of surface activation energy and frequency factor are dependent on the relative importance of each mode of combustion. In the two limits of reaction limited or diffusion limited cases. catalyst activity has little effect on the...

Arithmetic Mean Temperature Difference and the Concept of Heat Exchanger Efficiency

In this paper, it is shown that the Arithmetic Mean Temperature Difference, which is the difference between the average temperatures of hot and cold fluids, can be used instead of the Log Mean Temperature Difference (LMTD) in heat exchanger analysis. For a given value of AMTD, there exists an optimum heat transfer rate, Qopt , given by the product of UA and AMTD such that the rate of heat transfer in the heat exchanger is always less than this optimum value. The optimum heat transfer rate takes place in a balanced counter flow heat exchanger and by using this optimum rate of heat transfer, the concept of heat exchanger efficiency is introduced as the ratio of the actual to optimum heat transfer rate. A general algebraic expression as well as a chart is presented for the determination of the efficiency and therefore the rate of heat transfer for parallel flow, counter flow, single stream, as well as shell and tube heat exchangers with any number of shells and even number of tube passes per shell. In addition to being more intuitive, the use of AMTD and the heat exchanger efficiency allow the direct comparison of the different types of heat exchangers.Copyright

Analytical and Experimental Investigations of High Porosity Metallic Foams

A novel analytical model is developed to examine the temperature distribution of high porosity metallic foams. The foam is modeled as small fins, which results in a set of equations similar to those obtained in finite difference representation of the energy equation, from which foam temperature can be evaluated. These equations are solved simultaneously along with the energy equation for the fluid. The parameters needed to determine the performance of the foam are average strut length (dp ) and strut diameter (df ) which can be obtained from the knowledge of foam Porosity (e) and Pore Density (PPI). To validate the analytical model, the temperature distribution of an Aluminum 6101-T6 alloy foam with porosity 92% and pore density (10 PPI) is measured in a wind tunnel for different air velocities. There is close agreement between analytical and experimental results.Copyright

Urea Mixing in Selective Catalytic Reduction Systems

The tightening of emission standards mandates NOx and particulate emissions to be reduced by more than 90 percent by 2010 in Europe, United States, and Japan. Selective Catalyst Reduction (SCR) using Urea as the NOx reducing agent is fast becoming the preferred technology. This paper provides an overview of the state of art on the topic. It also examines the use of urea vapor instead of spraying an aqueous mixture and the impact of different spraying strategies on mixing. It is shown that by injecting urea vapor opposite to direction of the exhaust gas flow, better mixing with the exhaust and thus better conversion can be achieved as compared with injecting the urea vapor parallel to the gas. The increase in pressure drop does not appear significant.Copyright

Numerical Simulation of Heat Transfer in Simultaneously Developing Flows in Parallel Rectangular Ducts

The F correction factor charts, or the e-NTU relations all assume that the overall heat transfer coefficient is constant across the heat exchanger. The short lengths and the comparatively thick walls through which heat is conducted in microchannel heat exchangers preclude the existence of thermally fully developed flow over a large portion of the heat exchanger. In this study, a parallel flow heat exchanger is simulated numerically to determine the impact of different parameters on the performance and the accuracy of constant heat transfer coefficient assumption. It is shown that there is significant change in the overall heat transfer coefficient in the developing region and that the three-dimensional heat transfer in the heat exchanger wall must be included in the analysis.Copyright

The Shell and Tube Heat Exchanger Efficiency and Its Relation to Effectiveness

The heat exchanger efficiency is defined as the ratio of the actual heat transfer in a heat exchanger to the optimum heat transfer rate. The optimum heat transfer rate, qopt , is given by the product of UA and the Arithmetic Mean Temperature Difference, which is the difference between the average temperatures of hot and cold fluids. The actual rate of heat transfer in a heat exchanger is always less than this optimum value, which takes place in a balanced counter flow heat exchanger. It is shown that for parallel flow, counter flow, and shell and tube heat exchanger the efficiency is only a function of a single nondimensional parameter called Fin Analogy Number. Remarkably, the functional dependence of the efficiency of these heat exchangers on this parameter is identical to that of a constant area fin with an insulated tip. Also a general algebraic expression as well as a generalized chart is presented for the determination of the efficiency of shell and tube heat exchangers with any number of shells and even number of tube passes per shell, when the Number of Transfer Units (NTU) and the capacity ratio are known. Although this general expression is a function of the number of shells and another nondimensional group, it turns out to be almost independent of the number of shells over a wide range of practical interest. The same general expression is also applicable to parallel and counter flow heat exchangers.Copyright