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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

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Objectives Determine –Capillary Limit –Thermal Resistance –Evaporative Heat Transfer Coefficient Vary –Heat Input –Radial Acceleration –Fluid Inventory

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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

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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. 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

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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

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

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

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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)

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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)

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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

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Experimental Setup Thermocouple placement: Unheated/uncooled sections for thermal resistance Circumferential and axial distributions in evaporator section for evaporative heat transfer coefficient

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Experimental Results Temperature distributions: Uniform temps for low input power levels Evaporator temps increase with input power: Partial dryout of evaporator Inboard

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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

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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

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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)

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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)

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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)

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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

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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

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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

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