Thursday, December 15, 2011
Earlier post "Mal-Distribution of Phases at T-Junction Phenomenon" and "Stagnant Liquid In Inclined Parallel Pipe Downstream of T-Junction" have shown two phase flow mal-distribution and stagnant liquid phenomenon at T-junction or splitting tee.
The flow distribution from a manifold to parallel channels is becoming of interest in predicting the heat transfer performance of heat exchangers. As discussed, due to maldistribution of two phase flow at T-junction, flow rates through the channels to each heat exchanger are not uniform. In the extreme case, there is almost no flow through some of them (i.e. stagnant liquid phenomenon). As of today, there is still no general way to predict the distribution of two-phase mixtures at header–channel junctions (T-junction).
Simulation studies by Bernoux et al. (2001) with two-phase distribution at the inlet manifold of compact heat exchangers, results showed that the vapor distribution to the channels became uniform with the increase of the mass quality (Xv), but the liquid distribution still remained unbalanced. Besides, the liquid distribution through the channels was not much sensitive to the mass flux (MFlux).
Flow behavior studies (by Osakabe et al, 1999) in a horizontal square header (with each side being 40 mm) connected to four parallel vertical tubes (10 mm in diameter) for the bubbly and slug flows at the inlet, largest amount of the liquid flow into the first tube for a low mass quality (Xv) flow in the header; however, the tendency of mal-distribution is reduced with the increase of the mass quality (Xv) inside the header.
Flow behavior at one junction has no influence on that at the next junction. On the other hand, for the most of the compact heat exchangers, the distance between the channels is comparable to or even smaller than the header size (hydraulic diameter), flow interaction between the junctions has to be taken into account.
Infact, behavior of flow separation in small T-junctions appeared different from that in the large T-junctions. There are strong interaction between each T-junctions. The flow behavior at one T-junction is strongly influenced by other T-junction especially when the distance between the T-junction is equivalent to or smaller than the hydraulic diameter of the header (as reported by Lee & Lee, 2004).
Further investigation on two-phase flow behavior at the upward header co-current flow and horizontal rectangular parallel channels and simulating the corresponding parts of compact heat exchangers shown that intrusion depth of the channels into header affects flow distribution of the liquid-phase. Less amount of liquid was separated out through the channels at the rear part except near the end-partition with the zero intrusion depth, Reversed trend observed for deeper intrusion. This indicated that a uniform distribution could be obtained by adjusting the intrusion depth. Deeper intrusion channel promote mixing effect and hence the uniform distribution to each outlet.
Above shown that mal-distribution of two phase flow at T-junctions affects by many factors i.e. vapor mass quality, flow pattern, distance of T-junction, size of T-junction and header size, penetration of T-junction into header, etc. Infact, above only several factors affecting mal-distribution, there are many more factors affecting it. In a compact heat exchanger i.e. Plate Fin Heat Exchanger (PFHE), Brazed Aluminum Heat Exchanger (BAHX), gas & liquid fraction flow from header to each channel may be different from tube to tube. This could seriously affects the heat transfer performance of the heat exchanger. Not only that performance varies at each channel would lead to change in temperature profile, and hence thermal stress profile of heat exchanger which increase the fatigue cracking tendency and life span of the heat exchanger.
- Stagnant Liquid In Inclined Parallel Pipe Downstream of T-Junction
- Mal-Distribution of Phases at T-Junction Phenomenon
- Problems Caused by Two Phase Gas-Liquid Flow
- Slugging & Slugcatcher
- Liquid Slug Stabiliser - Another type of slugcatcher
- Assess Potential Piping Failure Due to Valve Quick Opening with Two-Phase Vapor Liquid
- How Fluid Characteristic affect 2 phase Relief via PSV on Liquid filled Vessel Exposing to External Fire