Experimental and computational investigation of gravity separation in a vertical flash tank separator

Experimental and computational investigation of gravity separation in a vertical flash tank separator

Vapour injection through flash tank separation is an effective way to enhance the coeffcient of performance (COP) and increase the capacity of air conditioning and refrigeration systems. In refrigeration systems which use the vapour injection technique, the flash tank feeds the evaporator with refrigerant liquid and the vapour is delivered to the compressor. Hence, the vertical flash tank is an important component that can be used to improve the performance of air conditioning and refrigeration systems. Semi-empirical design methods based on settling theory can be used for flash tank design, but the approach does not offer precise sizing data or an accurate assessment of likely performance under different operation conditions. Therefore, this thesis assesses the usefulness of CFD in
flash tank design, and this is achieved through experiments and simulations on a range of relevant configurations using water as the working fluid. A new experimental facility was built and five different models of the vertical separator were used to investigate the effect of the aspect ratio (H/D) on the liquid separation efficiency for a range of mass flow rates from 2.1 to 23.4 g/s. The internal diameter of the separator in each case was 50 mm. The separation tank was fed with two-phase flow generated using an expansion device and a length of horizontal pipe of diameter 25 mm. The two-phase flow developed gradually after the expansion device, reaching the developed region at about 165 cm downstream.

For the vertical tank without enhancement options, the highest value of the liquid separation efficiency of 0.96 was achieved using the highest height separator (H=250 mm) at the maximum mass flow rate 23.4 g/s. The separation efficiency increased with increasing inlet mass flow rate. In order to improve the liquid separation efficiency of the vertical separator, four enhancement options for the design were proposed: an extractor, a change in the inlet flow direction, a change in the position of the inlet pipe and a combination of the extractor and the change of the flow direction. The highest liquid separation efficiency of 0.99 was achieved by the last option at the maximum flow rate of 23.4 g/s. Empirical correlations were developed in horizontal pipe and vertical flash tank separator cases to predict the expansion length, void fraction and liquid separation efficiency. The range of the non-dimensional two-phase flow parameters covered by this study was: Reynolds number from 450.45 to 3246, Weber number from 0.005 to 0.72, and Froude number from 0.06 to 7.86.

CFD simulations were performed to simulate the two-phase flow within the horizontal tube which formed the inlet to the vertical separator, and the vertical separator itself. The simulations adopted the pressure based solver using the Eulerian-Eulerian multiphase model. In the horizontal tube case, the numerical results underestimated the expansion length in the experimental results with the simulated expansion length being an average of 8 % lower than the experimental observations. In the vertical tank separator case, the numerical results also underestimated the liquid separation efficiency in the experimental results with an average difference of the liquid separation efficiency of between 1 % and 2 %.

Based on the assessment of the CFD against the experiments of the present work, future applications of CFD in flash tank design should achieve liquid separation efficiency results that reflect the physical reality to within a few percent for operating conditions comparable to the present work. It is concluded that CFD can reliably be used for vertical flash tank design optimisation at least for stratified inlet flow conditions.