Offset Strip-fin Heat Exchanger Research Articles

A Complementary Experimental and Numerical Study of the Flow and Heat Transfer in Offset Strip-Fin Heat Exchangers

A detailed analysis of experimental and numerical results for flow and heat transfer in similar offset strip-fin geometries is presented. Surface-average heat transfer and pressure drop, local Nusselt numbers and skin friction coefficients on the fin surface, instantaneous flow structures, and local time-averaged velocity profiles are contrasted for a range of Reynolds numbers using both prior and new experimental and numerical results. This contrast verifies that a two-dimensional unsteady numerical simulation captures the important features of the flow and heat transfer for a range of conditions. However, flow three-dimensionality appears to become important for Reynolds numbers greater than about 1300, and thermal boundary conditions are important for Reynolds numbers below 1000. The results indicate that boundary layer development, flow separation and reattachment, wake formation, and vortex shedding are all important in this complex geometry.

Second Law Performance Analysis for Offset Strip-Fin Heat Exchangers

Abstract A design procedure for offset strip-fin (OSF) heat exchangers is proposed, on the basis of a minimum-entropy-production evaluation criterion. Proper dimensionless operational and geometric-parameters are introduced, and the thermodynamic impact of the OSF augmentation technique is Analyzed, for assigned heat exchanger working conditions, using available correlations for heat transfer and fluid friction coefficients. The proposed method allows the designer to determine the most suitable geometric configuration in order to achieve—under given operational constraints—the minimum dissipation effects of OSF heat exchangers.

A finite difference computational model of annular film-flow boiling and two-phase flow in vertical channels with offset strip fins

This paper describes a detailed computational model of annular two-phase flow through offset strip fin heat exchanger geometries. In this model, a modified finite difference scheme is used to compute the velocity and pressure fields throughout the vapor core of the fin matrix. The vapor phase model is iteratively matched to a second finite difference model of the liquid film flowing along the channel walls. Three-dimensional interactions between the liquid film flowing along the prime and secondary surfaces of the matrix are also incorporated into the model. By solving the Navier-Stokes equation in the vapor phase, this model provides, for the first time, detailed local information about the shear stress and static pressure variation along the liquid film. This makes it possible to study the local variation of the heat transfer coefficient and wall shear stress all along the matrix. The model is validated against experimental data for two different offset fin geometries and then used to systematically study the effects of fin and channel geometry on two-phase transport. Special properties of two-phase flow through offset fin matrices discovered using the model are also described and the effects of these properties on the accuracy of previous annular flow models is discussed.

Forced heat convection heat transfer from shrouded staggered fin arrays

The heat transfer characteristics of staggered, frontally shrouded fin arrays have been studied experimentally, in the case of forced convection of air. Nusselt number and Colburn number are given, for the range of Reynolds numbers from 2000 to 12000. The local Nusselt number distributions signal the existence of the periodically developed temperature field. Average Colburn number values are found to be in agreement, from an engineering standpoint, with correlations predicted for offset strip-fin heat exchangers.

Heat transfer and friction in the offset stripfin heat exchanger

This paper presents analytical models to predict the heat transfer coefficient and the friction factor of the offset strip-fin heat exchanger surface geometry. Two flow regimes are defined—laminar and turbulent. Based on the conditions in the wake, an equation is developed to predict transition from laminar to turbulent flow. Flow visualization experiments were performed to identify the flow structure at transition. The condition predicted by the transition equation corresponds to onset of oscillating velocities in the fin wakes. Equations are developed for the Nusselt number and friction factor by writing energy and momentum balances on a unit cell of the offset strip-fin geometry. A numerical solution is used to calculate Nu and f on the fins in the laminar regime, and a semi-empirical approach is used for the turbulent regime. Predictions are compared to data on scaled-up geometries, taken in the present study, and data on actual heat exchangers. The models predict all data within ±20%.