Wenhai Li

Analysis of Interacting, Underexpanded, Rarefied Jets

A numerical study, using the Direct Simulation Monte Carlo (DSMC) approach, of the interaction effects between two identical sonic under-expanded nitrogen jets under rarefied conditions is reported. In this paper, the effects of the jet stagnation Knudsen number (Kns), the ratio of the stagnation-to-background pressure (Ps/Pb), and the distance between the jet orifices (L/D) were investigated for a range of these parameters. The background pressure has a very significant effect on the physics of flow. The value of L/D affects the locations of the Mach disk in both primary and secondary jets. But When Ps/Pb is relatively large (such as Ps/Pb=220) and L/D is relatively small (L/D≤6.0), the separation distance L/D has only a mild effect on the location of the Mach disk in the secondary jet. A study of the rotationaltranslational non-equilibrium showed that large deviations between the translational and rotational temperatures can be found in the secondary jet, especially in the vicinity of the orifice plate.

Comparison of 30 Boiling and Condensation Correlations for Two-Phase Flows in Compact Plate-Fin Heat Exchangers

Over the years, empirical correlations have been developed for predicting saturated flow boiling [1-15] and condensation [16-30] heat transfer coefficients inside horizontal/vertical tubes or micro-channels. In the present work, we have examined 30 of these models, and modified many of them for use in compact plate-fin heat exchangers. However, the various correlations, which have been developed for pipes and ducts, have been modified in our work to make them applicable to extended fin surfaces. The various correlations have been used in a low-order, one-dimensional, finite-volume type numerical integration of the flow and heat transfer equations in heat exchangers. The NIST’s REFPROP database [31] is used to account for the large variations in the fluid thermo-physical properties during phase change. The numerical results are compared with Yara’s experimental data [32]. The validity of the various boiling and condensation models for a real plate-fin heat exchanger design is discussed. The results show that some of the modified boiling and condensation correlations can provide acceptable prediction of heat transfer coefficient for two-phase flows in compact plate-fin heat exchangers. INTRODUCTION In recent years, increased efforts have been devoted to the design of high performance compact heat exchangers due to the increasing heating and cooling requirements in various thermal systems. Among the many enhanced heat transfer techniques, working fluid with phase change by boiling or condensation can provide large heat fluxes, even with relatively small driving temperature differences. Therefore, two phase flows are widely used in the design of heat exchangers. Many efforts have been devoted to understanding the basic phase-change in the past and to develop models to predict heat transfer coefficients [33-36]. Multiple correlations for fluid boiling or condensation heat transfer have been proposed in the literature, most of them having been developed empirically. However, as for the design of the compact heat exchangers with two phase flows, the available boiling and condensation correlations cannot be used directly since these correlations were originally developed for two-phase flows in horizontal/vertical smooth tubes or, recently, for flows in micro-channels. The accuracy of these correlations for two-phase flows in compact heat exchangers need to be tested due to more complicated flow passage geometries and the existence of extended surfaces such as the fins in the plate-fin heat exchangers. In this study, 15 boiling and 15 condensation correlations taken from the literature are chosen to predict the boiling and condensation heat transfer in compact plate-fin heat exchangers. The existing models developed have been modified to account for the effects of fins. The proposed modified models are then used to predict the heat transfer for R22 boiling and condensation inside a compact plate-fin heat exchanger with serrated fins. The numerical results are compared with Yara’s experimental data, so that the validity of these modified models can be assessed. BOILING CORRELATIONS Many correlations have been proposed for predicting the heat transfer coefficient in situations where a liquid boils. The 15 boiling correlations [1-15] tested in this study are summarized in Table 1. For use in a plate-fin heat exchanger, the existing boiling correlations need to be modified so that the effects of the fins to both nucleate and convective boiling can be taken into account. Several studies [37, 38] have been carried out to study the boiling phenomena in a compact platefin evaporator. It was found that, on one hand, similar to single phase flow, the existence of the fins may lead to larger Reynolds number so that the convective boiling can be

Analysis of Interacting, Under-Expanded, Rarefied Jets

A numerical study, using the Direct Simulation Monte Carlo (DSMC) approach, of the interaction effects between two identical sonic under-expanded nitrogen jets under rarefied conditions is reported. In this paper, the effects of the jet stagnation Knudsen number (Kns), the ratio of the stagnation-to-background pressure (Ps/Pb), and the distance between the jet orifices (L/D) were investigated for a range of these parameters. The background pressure has a very significant effect on the physics of flow. The value of L/D affects the locations of the Mach disk in both primary and secondary jets. But When Ps/Pb is relatively large (such as Ps/Pb=220) and L/D is relatively small (L/D≤6.0), the separation distance L/D has only a mild effect on the location of the Mach disk in the secondary jet. A study of the rotationaltranslational non-equilibrium showed that large deviations between the translational and rotational temperatures can be found in the secondary jet, especially in the vicinity of the orifice plate.