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 Pipe Straight Heat Pipe RR ω ω

Previous Research Klasing, 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 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 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 m t = m v + m l = V vs /v v + GV gr /v l Total Inventory Mass V vs = πD vs 2 L t /4 + V gr (1 - G)Vapor Space Volume V gr = L gr N gr A gr Groove Volume A gr = wh + h 2 (tan θ 1 + tan θ 2 ) /2 Groove Area L gr = L t [(2πr h /p) 2 + 1] 1/2 Groove Length p = 2π(s - s 1 )/(φ - φ 1 )Groove Pitch G = V l /V g Ratio of Liquid Volume to Total Groove Volume

Working Fluid Inventory Measure groove height and width –Bitmap image from microscope –Microscope scale –Adobe Illustrator w h θ1θ1 θ2θ2

Working Fluid Inventory Measure helical groove pitch –Angular transducer –High precision voltmeter –Vertical milling machine

Working Fluid Inventory θ1θ1 h w θ2θ2 V - V 1 s - s 1 D vs LtLt A gr p rhrh L gr V gr G m t (g)

Heat Pipe Fill Station No horizontal lines Short runs of large diameter tubing Detect and remove trapped vapor by cycling valves Fully calibrated G Δm t (g)Δm d (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, R th decreased with a r For G = 1.0 and 1.5, R th decreased and then increased when dryout commenced For G = 1.5, dryout was not reached for a r > 2.0-g G = 0.5 G = 1.0 Q t (W) G = 1.5

Experimental Results Evaporator temperature vs transported heat for a r = 0.01-g: Temperature increased with Q t 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 Q t (W) x = 54 mm x = 92 mm x = 130 mm x = 168 mm

Experimental Results Evaporator temperature vs transported heat for a r = 10.0-g: Dryout was delayed due to improved pumping of helical grooves Temperature variation around circumference was greater than a r = 0.01-g Q t (W) x = 54 mm x = 92 mm x = 130 mm x = 168 mm Q t (W)

Experimental Results Evaporative heat transfer coefficient vs transported heat for a r = 0.01-g: h e was very low for G = 0.5 due to dryout h e increased and then decreased as dryout was approached For G = 1.0, partial dryout along the axis occurred (h e converged around circumference) Q t (W) x = 54 mm x = 92 mm x = 130 mm x = 168 mm Q t (W)

Experimental Results Evaporative heat transfer coefficient vs transported heat for a r = 10.0-g: h e was more uniform around the circumference and along the axial direction for G = 1.0 h e was more constant with respect to Q t compared with a r = 0.01-g Q t (W) x = 54 mm x = 92 mm x = 130 mm x = 168 mm Q t (W)

Comparison of Analytical Capillary Limit Model and Experimental Data Maximum heat transport vs radial acceleration: Q cap increased significantly with a r For G = 0.5, heat pipe operated only for a r  8.0-g For G = 1.5, capillary limit could not be reached for a r  4.0-g Analytical model agrees well with data for G = 1.0 –Assumed full grooves, no liquid communication a r (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