Stephen Idem

Sensible and Latent Heat Transfer to a Baffled Finned-Tube Heat Exchanger

A straightforward heat exchanger thermal model is presented and verified experimentally. It may be used to predict the performance of a coil that has condensation occurring on the gas side. Experimentally determined heat and mass transfer coefficients are employed in the model Sample data are presented for a wide range of surface wetting conditions. Partially wet heat exchangers are analyzed by employing an area weighting scheme to the dry and wet portions of the coil with moderate success. It is shown that employing a heat / mass transfer analogy in predicting the performance of a condensing heal exchanger may lead to a considerably undersized coil design.

Predicting the Performance of Multistage Heat Exchangers

A simple method for predicting the thermal performance of multistage heat exchangers is proposed. The technique involves an area weighting scheme of the reseated Colburn J factors. The method is developed for, and probably limited to, applications with indirect contact, direct transfer heat exchangers operated in series. However, condensing heat exchangers are amenable to this technique, and thus it offers more flexibility and wider applicability than previously proposed methods. Experimental data for a two-stage, condensing heat exchanger are reported, and the behavior is compared to the predictive scheme with moderate success. While the behavior of multistage heat exchangers may be affected by interstage spacing, the data from the experiments do not allow quantification of the impact of this effect.

Pressure Loss Coefficient Measurements of Two Close-Coupled HVAC Elbows

An experimental program was initiated to determine the friction factor in a straight duct and the loss coefficient of either a single elbow or two elbows placed in close proximity. The tests were performed in accordance with ANSI/ASHRAE Standard 120-1999. Loss coefficients C were measured for six kinds of single elbows. Likewise, identical pairs of elbows were considered in the close-coupled elbow experiments. For long separation lengths, the loss coefficient for two close-coupled elbows approached to within approximately 10% of the value of 2*C for a single elbow, for most cases considered in this study. The single-elbow data were also compared with those published in the 2002 ASHRAE Duct Fitting Database

Influence of Area Ratio On Flat Oval Divided-Flow Fitting Loss Coefficients

A method of correlating branch loss coefficient data as a function of branch-to-common flow rate and area ratios is described. The algebraic expression that was used to successfully collapse the data from a given fitting geometry class onto a single curve was based on physical reasoning, and cannot be anticipated by conventional scaling arguments. The coefficients of empirical correlations were derived from previously published data and are tabulated for straight-body and conical tees and laterals. For flat oval fittings at a given flow rate ratio, as the area ratio increased the branch loss coefficient increased. At a given flow rate ratio, straight-body branches typically exhibited higher loss coefficients than conical branches, for the same flat oval branch-to-common area ratio. Tees consistently displayed higher branch loss coefficients than did laterals for the same flat oval branch/common geometry.

Transient Performance of a Cross Flow Heat Exchanger Using Finite Difference Analysis

Transient analysis of a cross flow heat exchanger subjected to a step increase in temperature of the minimum capacity rate fluid was considered. Predictions obtained using an implicit central finite difference method were presented. The influence of varying grid sizes and time steps sizes on these approaches was addressed. A parametric study was conducted by varying the dimensionless parameters governing the transient response of heat exchanger over a representative range of values.© 2008 ASME

A General Matrix Approach to Model Steady-State Performance of Cross-Flow Heat Exchangers

A steady-state performance model of multirow multipass cross-flow tubular heat exchangers is developed. The proposed matrix approach uses the concepts of local effectiveness, energy balance, and number of transfer units (NTU) applied to every pass/row in the cross-flow heat exchanger to predict thermal performance. The method can predict the total effectiveness of assemblies of heat exchangers. Several circuiting configurations, such as overall counter-cross-flow, overall parallel cross-flow, and fluids in parallel in one of the streams, were considered. Predictions of the steady heat transfer performance of selected multirow multipass cross-flow heat exchangers are obtained by applying the general matrix approach. The heat exchanger geometries selected for the comparative study represent common cross-flow heat exchanger configurations used in industry. For these heat exchangers the overall heat exchanger effectiveness values were computed for various capacity rate ratios and NTU values. The validity of the matrix approach was then verified by comparing the resulting predictions with those obtained using the P-NTU approach and the Domingos method for the selected complex cross-flow heat exchanger configurations.

Laboratory testing of converging flow flat oval tees and laterals to determine loss coefficients (RP-1488)

Pressure loss coefficients were measured for several converging flow straight body flat oval tees and laterals in accordance with ASHRAE Standard 120-2008 (ASHRAE 2008). The aspect ratio for both main and branch ducts was varied from 2.2 to 3.8 using combinations of three main duct sizes and four branch duct sizes. A logarithmic model was used to correlate branch and main loss coefficients as functions of branch-to-common and main-to-common flow rate ratio, respectively, and pertinent geometry characteristics of the fitting. Uncertainty analysis was performed on the measured experimental data. Coefficient of determination values ranged from 0.60 to 0.82.

On-Line Performance Model of the Convection Passes of a Pulverized Coal Boiler

This article describes an on-line heat transfer simulation for the convection passes of a typical pulverized coal boiler (PCB) power plant that accounts for fouling. Performance analysis of heat exchanger assemblies employed in pulverized coal boilers was characterized using the effectiveness–number of transfer units (NTU) method. The model calculates instantaneous heat rates in different sections of the boiler so as to determine a local cleanliness factor. The effects of changing plant load are fully accounted for in the model. Generally, a close correlation between calculated cleanliness factors and normalized strain gage measurements of pendant section weight variations due to accumulated fouling was obtained. Furnace exit gas temperatures calculated by the model agreed reasonably well with measurements available in the literature for a similar design of PCB power plant.

An improved 1D fiber dry spinning mass transfer model

Abstract A method of extending one-dimensional models of fiber dry spinning to take differences between surface and average mass concentrations into account is evaluated using an idealized model problem. Based on a closed form solution of the model problem, it is concluded that the method (which involves the introduction of a separate surface concentration variable and a corresponding additional equation) works well.

Experimental determination and computational fluid dynamics predictions of pressure loss in close-coupled elbows (RP-1682)

An experimental program was implemented to study pressure losses in HVAC duct systems associated with 305 mm (12 in.) diameter close-coupled round five-gore elbows. The goal of this program was to experimentally verify a computational fluid dynamics model to accurately predict pressure losses in order to design duct systems more effectively. The results of this study showed that the loss coefficient increased as a function of separation distance between the elbows in a Z-configuration and decreased in a U-configuration. For both 305 mm (12 in.) and 203 mm (8 in.) diameter elbows, power law expressions correlating the combination loss coefficient data as a function of intermediate length for close-coupled elbows arranged in a Z-configuration or a U-configuration were presented. Computational fluid dynamics modeling with enhanced wall treatment using the k-ϵ method was generally able to correctly predict elbow loss coefficients with an error of less than 15%.