Using a modified single-phase model to predict microgravity flow boiling heat transfer in the bubbly flow regime

ABSTRACT

It is hypothesized that the heat transfer in microgravity bubbly flow boiling can be computed through a single-flow simulation that accounts for the acceleration of the liquid as bubbles form since the slip velocity in microgravity is negligible. Measurements within the bubbly flow regime were obtained in a 6 mm ID sapphire tube in microgravity using HFE-7000 at four mass fluxes, six heat fluxes, and two subcoolings at atmospheric pressure. Flow visualization was performed and time and space resolved temperature and heat transfer distributions at the wall–fluid interface were measured using a Temperature Sensitive Paint (TSP) applied to the inside of the tube. The local liquid velocity was determined from the movement of small bubbles in the flow. The local, time-averaged heat transfer data were compared to numerical simulations of single-phase flow in a tube whose diameter was varied to match the experimentally obtained local liquid velocity. When the flow within the tube was laminar (low heat flux and mass flux cases), the measured heat transfer agreed well with the numerical results. For cases where the flow became transitional/turbulent and significant bubble coalescence was present, the measured heat transfer was higher, but was bounded by numerical solutions assuming laminar and turbulent flow.

KEYWORDS:

• Temperature sensitive paints
• phase change
• flow boiling
• gravity
• heat transfer coefficient

Acknowledgments

Support for this research was provided through NASA Grant NNX09AK39A. Mr. Caleb Hammer was funded on a NASA NSTRF Fellowship through Grant NNX16AM94H. The low-gravity flights were supported through the NASA REDDI program through Grant NNX16AP62G.

Declaration of interest statement

none.

Nomenclature

• A_c,b = Bubble Cross section (m²)

• C_D = Drag coefficient (dimensionless)

• C_p = Specify heat capacity (J/kg.K)

• D = Diameter (m)

• E = Emission intensity (bits)

• F = Force (N)

• g = Gravitational acceleration (m/s²)

• G = Mass flux (kg/m².s)

• h = Heat transfer coefficient (W/m².K)

• I = Turbulence intensity (dimensionless)

• k = Thermal conductivity (W/m.K)

• K = Specific turbulent kinetic energy (m²/s²)

• l = Turbulence length scale (m)

• m = Mass (kg)

• n = Number of points

• r = Radius (m)

• Re = Reynolds number (Re=vD/ν) (dimensionless)

• q” = Heat flux (W/m²)

• T = Temperature (°C)

• v = Velocity (m/s)

• z = Distance along tube from tube inlet (mm)

Greek

• ΔT = Temperature difference (°C)

• ε = Turbulent dissipation rate (m²/s³)

• μ = Dynamic viscosity (Pa.s)

• ν = Kinematic viscosity (m²/s)

• ρ = Density (kg/m³)

• σ = Surface tension (N/m)

• τ = time constant (s/s) (dimensionless)

• ω = Specific turbulent dissipation rate (1/s)

Subscripts

• 0 = Initial

• 1g = Terrestrial gravity upward

• b = Bubble

• B = Buoyancy

• bulk = Bulk

• exp = Experimental

• D = Drag

• i = Tube inlet

• lam = Laminar

• l = Liquid

• m = Mean

• max = Maximum

• num = Numerical

• Q = Constant heat flux

• sat = Saturation

• sub = Subcooling

• str = Straight Tube

• t = Terminal

• T = Constant temperature

• μg = Microgravity

• v = Vapor

• + = Upper error

• - = Lower error

Additional information

Funding

This work was supported by the National Aeronautics and Space Administration [NNX09AK39A, NNX16AM94H, NNX16AP62G].