The Effect of Working Fluid Inventory on the Performance of Revolving Helically-Grooved Heat Pipes Presenter: Dr. Scott K. Thomas, Wright State University Co-authors: R. Michael Castle, Graduate Research Assistant (Currently with Belcan Corp.) Dr. Kirk L. Yerkes, AFRL/PRPG
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The Effect of Working Fluid Inventory on the Performance of Revolving Helically-Grooved Heat Pipes
The Effect of Working Fluid Inventory on the Performance of Revolving Helically-Grooved Heat Pipes. Presenter: Dr. Scott K. Thomas, Wright State University Co-authors: R. Michael Castle, Graduate Research Assistant (Currently with Belcan Corp.) Dr. Kirk L. Yerkes, AFRL/PRPG. Objectives. - PowerPoint PPT Presentation
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The Effect of Working Fluid Inventory on the Performance of Revolving Helically-Grooved
Heat Pipes
Presenter:
Dr. Scott K. Thomas, Wright State University
Co-authors:
R. Michael Castle, Graduate Research Assistant
(Currently with Belcan Corp.)
Dr. Kirk L. Yerkes, AFRL/PRPG
Objectives
• Determine
– Capillary Limit
– Thermal Resistance
– Evaporative Heat Transfer Coefficient
• Vary
– Heat Input
– Radial Acceleration
– Fluid Inventory
Applications of Revolving Heat Pipes
• Thermal Management of Rotating Devices– Aircraft Generators
– Large-Scale Industrial Electric Motors
– Rotating Satellites
Curved Heat PipeStraight Heat Pipe
R R ω
ω
Previous ResearchKlasing, K., Thomas, S., and Yerkes, K., 1999, “Prediction of the Operating Limits of Revolving
Helically-Grooved Heat Pipes,” ASME Journal of Heat Transfer, Vol. 121, pp. 213-217.
Thomas, S., Klasing, K., and Yerkes, K., 1998, “The Effects of Transverse Acceleration-Induced Body Forces on the Capillary Limit of Helically-Grooved Heat Pipes,” ASME Journal of Heat Transfer, Vol. 120, pp. 441-451.
Findings:
• Capillary limit increased significantly with radial acceleration
• Straight axial grooves showed no improvement with radial acceleration
Shortcomings:
• Effect of liquid fill not examined
• Helical groove geometry not rigorously determined
Working Fluid Inventory
mt = mv + ml = Vvs/vv + GVgr/vl Total Inventory Mass
Vvs = πDvs2Lt/4 + Vgr(1 - G) Vapor Space Volume
Vgr = LgrNgrAgr Groove Volume
Agr = wh + h2(tan θ1 + tan θ2) /2 Groove Area
Lgr = Lt[(2πrh/p)2 + 1]1/2 Groove Length
p = 2π(s - s1)/(φ - φ1) Groove Pitch
G = Vl/Vg Ratio of Liquid Volume to Total Groove Volume
Working Fluid Inventory
• Measure groove height and width– Bitmap image from
• Detect and remove trapped vapor by cycling valves
• Fully calibrated
0.5 3.6% 5.0%1.0 3.7% 2.9%1.5 3.6% 1.9%
G Δmt (g) Δmd (g)
Experimental Setup
• 8 ft dia Centrifuge Table
• 20 HP DC motor
• Separate instrumentation and power slip rings
• On-board TC signal conditioning
• Double-pass hydraulic rotary coupling
• Copper-ethanol heat pipe bent to outer radius of centrifuge table
Experimental Setup
Thermocouple placement:
• Unheated/uncooled sections for thermal resistance
• Circumferential and axial distributions in evaporator section for evaporative heat transfer coefficient
Experimental Results
Temperature distributions:
• Uniform temps for low input power levels
• Evaporator temps increase with input power: Partial dryout of evaporator
Inboard
Experimental Results
Thermal resistance vs transported heat:
• For G = 0.5, partial dryout even for low power, Rth decreased with ar
• For G = 1.0 and 1.5, Rth decreased and then increased when dryout commenced
• For G = 1.5, dryout was not reached for ar > 2.0-g
G = 0.5
G = 1.0
Qt (W)
G = 1.5
Experimental Results
Evaporator temperature vs transported heat for ar = 0.01-g:
• Temperature increased with Qt
• For G = 1.0, grooves were full near adiabatic section, dry near evaporator end cap
• Temps converge to the same value around the circumference during dryout
Qt (W)
x = 54 mm
x = 92 mm
x = 130 mm
x = 168 mm
Experimental Results
Evaporator temperature vs transported heat for ar = 10.0-g:
• Dryout was delayed due to improved pumping of helical grooves
• Temperature variation around circumference was greater than ar = 0.01-g
Qt (W)
x = 54 mm
x = 92 mm
x = 130 mm
x = 168 mm
Qt (W)
Experimental Results
Evaporative heat transfer coefficient vs transported heat for ar = 0.01-g:
• he was very low for G = 0.5 due to dryout
• he increased and then decreased as dryout was approached
• For G = 1.0, partial dryout along the axis occurred (he converged around circumference)
Qt (W)
x = 54 mm
x = 92 mm
x = 130 mm
x = 168 mm
Qt (W)
Experimental Results
Evaporative heat transfer coefficient vs transported heat for ar = 10.0-g:
• he was more uniform around the circumference and along the axial direction for G = 1.0
• he was more constant with respect to Qt compared with ar = 0.01-g
Qt (W)
x = 54 mm
x = 92 mm
x = 130 mm
x = 168 mm
Qt (W)
Comparison of Analytical Capillary Limit Model
and Experimental Data
Maximum heat transport vs radial acceleration:
• Qcap increased significantly with ar
• For G = 0.5, heat pipe operated only for ar 8.0-g
• For G = 1.5, capillary limit could not be reached for ar 4.0-g
• Analytical model agrees well with data for G = 1.0
– Assumed full grooves, no liquid communicationar (g)
ar (g)
G = 0.5
G = 1.0
G = 1.5
Conclusions
• Capillary limit increased, thermal resistance decreased significantly with working fluid inventory
• Evaporative heat transfer coefficient was a strong function of working fluid inventory
• Analytical model prediction was good for G = 1.0, but unsatisfactory for underfilled and overfilled heat pipes
Current Research• Thomas, S., Lykins, R., and Yerkes, K., 2000, "Fully-Developed Laminar Flow in Trapezoidal Grooves with Shear
Stress at the Liquid-Vapor Interface," submitted to the International Journal of Heat and Mass Transfer.• Thomas, S., Lykins, R., and Yerkes, K., 2000, "Fully-Developed Laminar Flow in Sinusoidal Grooves," submitted
to the ASME Journal of Fluids Engineering.
• Use results of numerical model to improve analytical capillary limit model for revolving helically-grooved heat pipes
• Numerical model accounts for countercurrent liquid-vapor shear stress and working fluid inventory