Interactions Between Parallel Unevenly Heated Minichannels During Flow Boiling of R134A
Transient pressure drop of individual channels during flow boiling of R134a in four 0.54 mm square parallel minichannels was experimentally studied in this work. The design of the test section enabled the experimenter to control and to vary heat flux independently in each channel in the range from 3.82 to 18.66 kW/m2 at five different overall flow rates from 86 to 430 kg/m2 s. Flow rate fluctuation in parallel channels due to the formation of bubbles under the nonuniform heat flux conditions caused significant oscillations in local pressure drop. Statistical analysis indicated that the pressure drop signal was normally distributed when boiling was stable with no incoming flow disturbance. Pressure drop distribution was highly skewed and multimodal when significant evaporation rate at low mass fluxes led to rapid annular flow formation, reducing the free flow of incoming fluid. Cross correlation analysis revealed a strong interaction between minichannels having the highest heat flux difference among the set of channels. The least heated channel was more sensitive to the fluctuations in other channels. Cross correlation between the most heated channel and the adiabatic one was estimated to be 39% when the total flow rate was the lowest, 86 kg/m2 s. The power of the relationship between channels dropped significantly as the flow rate increased. Less than 5% of data points could be considered cross correlated at the highest flow rate of 430 kg/m2 s. Increasing the two phase pressure drop across each channel caused higher resistance to the incoming disturbances and led to less interchannel interaction. This study of the channels interaction in a system of parallel, nonuniformly heated minichannels can be used as a tool to identify and quantify instabilities and reversed flow conditions.