During my graduate studies at Berkeley, I studied heat transfer with phase change. One of our projects was a multi-physics model of pipe flow through an evaporator.
*Images, parameters and equations from Prof. Van Carey’s Course Project Assignment
Pipe flow with phase change in a vertical pipe goes through various flow regimes that affect the heat transfer properties of the flow. Flow regimes can be depicted to the left in the image.
For this project, we use a 1-D separated flow model to predict the pipe's pressure drop, void fraction, and wall pipe temperature.
Using a 1-D separated flow model, we predicted the pressure drop and heat transfer performance for flow boiling of R-134a. We interpolated chart data of R-134a that
We used our program to predict the pressure and wall temperature variation along the tube for the following conditions of flow boiling of R-134a:
Tube wall thickness tw = 2.0 mm
Tube length Lt = 1.2 m
Total rate of heat transfer over 1.2 m tube = 360.0 W
Vertical tube inlet design conditions: P = 388 kPa, Tsat= 8.0 °C, x = 0.15
Set the refrigerant inlet mass flow at the value that will make the exit quality = 0.95
We used our program to predict the pressure and wall temperature variation along the tube for the following conditions of flow boiling of R-134a:
Tube inside diameter Di = 6.0 mm
Tube wall thickness tw = 2.0 mm
Tube length Lt = 1.2 m
Total rate of heat transfer over 1.2 m tube = 360.0 W
Vertical tube inlet design conditions: P = 388 kPa, Tsat= 8.0 °C, x = 0.15
Set the refrigerant inlet mass flow at the value that will make the exit quality = 0.95
We broke up the tube into segments small enough that the change in quality across each segment is less than 0.01 and determined parameters shown in the table and plots below. Table 1 shows the mass flow rate, mass flux, and overall fluid pressure drop down the pipe.
At a pipe length of 0.42m the rate of change of temperature changes. This is due to the change in heat transfer coefficient. At a pipe length of 0.42m, we see a change from nucleate boiling regime to convective boiling regime.
We used Kandlikar’s correlation to determine the heat transfer coefficient. At 0.42m, we see a change in the rate of change of the heat transfer coefficient. This is where we jump from nucleate boiling dominant to convective boiling dominant.