A study of heat transfer, pressure drop and residence time distribution for two-phase, two component flow in a plate and frame heat exchanger
This thesis presents a literature review and the results of the present investigation for the upward flow of mixtures of air and water, water or air only in the channel of two Alfa-Laval plate heat exchangers. The flow patterns and their areas in the channel were identified photographically and visually. For both single-phase and two-phase flow, the embossed section of the channel was occupied by the slow and fast zones of flow. The ratio of the areas of these zones varied from nearly one to negligibly small as the flow rate of the phase(s) was increased gradually. These data were correlated empirically with the flow variables of the phase(s). High speed photography was used to analyse mixing in the channel as well as each cell formed by the herringbone patterns of the plates. The data on the flow patterns were used to develop a mathematical model of mixing in the channel to predict the exit concentration profiles of the phase(s). The residence time distribution of the phase(s) predicted by the present model compared very well with the experimental data for both singlephase and two-phase flow of the phases. However, the fit between the experimental and predicted exit concentrations was exceptionally good at high flow rates of the phase(s). The fractional volume of air occupied in the mixture of two-phase flow derived from the mean residence time of twophase flow were correlated empirically with dimensionless groups. The mixing parameters of the gamma distribution model were also correlated with the single-phase and two-phase flow dimensionless groups. The mean heat transfer film coefficients of water (hsL) and mixtures of air and water (htp) for the clean plates were correlated empirically. A Nusselt type equation was used to correlate the mean heat transfer film coefficients for water. The ratio (htp/hsL) was also correlated empirically against single-phase and two-phase dimensionless groups by two correlations which suggest the presence of two widely different flow patterns and mixing characteristics in the channel. Within the range of the variables investigated the values of htp increased by 53 to 340 percent over those for hsL. The pressure drop data for both metal and perspex plate heat exchangers were analysed with the Fanning friction factor (fr) v. Reynolds Number of water or air type of relationships. For air and water two different curves were obtained for the fr: Re relationships. For the analysis of the two-phase pressure data, an empirical approach incorporating the use of the dimensionless groups viz. the ratio of the liquid to gas superficial Reynolds Numbers and the Lockhart & Martinelli parameter (XLM) was found to correlate the twophase friction factor data extremely well. Finally, the pressure drop data were correlated equally well by a Lockhart & Martinelli (12) plot similar to that presented for the tubes.