Nakhlé Kattan

Flow Boiling in Horizontal Tubes: Part 1—Development of a Diabatic Two-Phase Flow Pattern Map

An improved two-phase flow pattern map is proposed for evaporation in horizontal tubes. The new map was developed based on flow pattern data for five different refrigerants covering a wide range of mass velocities and vapor qualities. The new map is valid for both adiabatic and diabatic (evaporating) flows and accurately identifies about 96% of the 702 data points. In addition, the new flow pattern map includes the prediciton of the onset of dryout at the top of the tube during evaporation inside horizontal tubes as a function of heat flux and flow parameters.

Flow Boiling in Horizontal Tubes: Part 3—Development of a New Heat Transfer Model Based on Flow Pattern

A new heat transfer model for intube flow boiling in horizontal plain tubes is proposed that incorporates the effects of local two-phase flow patterns, flow stratification and partial dryout in annular flow. Significantly, the local peak in the heat transfer coefficient versus vapor quality can now be determined from the prediction of the location of onset of partial dryout in annular flow. The new method accurately predicts a large, new database of flow boiling data, and is perticularly better than existing mehods at high vapor qualities (x > 85%) and for stratified types of flows.

Flow Boiling in Horizontal Tubes. Part 2; New Heat Transfer Data for Five Refrigerants

A summary of a comprehensive experimental study on flow boiling heat transfer is presented for five refrigerants (R134a, R123, R402A, R404A and R502) evaporating inside plain horizontal, copper tube test sections. The test data were obtained for both 12.00 mm and 10.92 mm diameters using hot water as the heating source. Besides confirming known trends in flow boiling heat transfer data as a function of test variables, it was also proven that the heat flux level at the dryout point at the top of the tube in annular flow has a very significant downstream effect on heat tranfer coefficients in the annular flow regime with partial dryout.