1. 1 STEADY-STATE AND TRANSIENT LOOP HEAT PIPE PERFORMANCE DURING PERIODIC HEATING CYCLES D. Mishkinis, G. Wang, D. Nikanpour, Materials and Thermal Group, Spacecraft Engineering, Canadian Space Agency St-Hubert, Quebec E. MacDonald, and T. Kaya Carleton University, Mechanical Aerospace Engineering, Ottawa, Ontario

2. 2 New Reliable & Robust LHP Thermal Control MODELS & THERMAL MODELING TOOLS, DEVICES & SYSTEMS New Engineering & Scientific Problems & Challenges... ; LHP: Start-up & Transient Operation, Temp-Control,..., New Thermal Control Solutions: Loop Heat Pipe Technology,... Extremely High Heat Flux Densities + Variable Heat Loads + Precise Temperature Control + Autonomy in Thermal Control INTRODUCTION : LHP TECHNOLOGY AND SPACE APPLICATIONS New Compact & Flexible Technologies Advanced Spacecraft

3. 3 1. Liquid & Vapor Distribution in Ev & CC: Working Fluid Inventory Prehistory: Pre-Shut-Down, Preconditioning, Manipulation Orientation in Gravity (or other g) Field 2. LHP Elements (Ev, Con, LL, VL, CC, 1 st & 2 nd Wicks) Mass, Geometry, Design & Material Properties; 3. Heat Input Power & Heat Sink Temperature; 4. Thermophysical & Chemical Properties of Fluid; 5. Environment Conditions; 6. Non-condensed Gas Presence & Amount; 7. Evaporator (Payload) Mass INTRODUCTION : LHP START-UP DEPENDS ON 4 Situations (Maidanik, 1995)

4. 4 INTRODUCTION : LHP START-UP INITIATION (  ) & T-CONTROL (  ) LHP T-Control Devices Classification (Nikitkin et al., 1999) NO External Q to LL or/& CC (Internal Q Redistribution) YESA) + B) Controlled Valve between VL & CC ( Nikitkin )  Mechanical Valve on VL / LL ( Anderson )  Three-way Valve on LL / VL ( Nikitkin )  Advanced LHP ( Hoang )  Heater on CC or LL  Heater on CC+VCHP links VL & LL ( Bienert ) TEC on CC with Q Link to EV (Khrustalev)  T-Control Valve on LL ( Gottschlich )  Pressure Valve between VL & CC and with Q Link to EV ( Goncharov )  Q Link (HP) between LL & VL ( Goncharov )  Q Link (HP) between CC & Environment ( Goncharov )   ONLY Small High Heat Flux Heater on EV (Krotiuk) Heater on VL (Gottschlich) Capillary Starter Pump (Ku)

5. 5 I.PRE-CONDITIONING: Heating CC to at least 10 K above the Evaporator for a min duration of 0,5 hour (Wrenn et.al., 2000). Heating CC by 10 W heater for 10 min at the end of each test. (Rodrigues et.al., 2000). Cooling of Condenser to the desired set point T (Ku et.al., 2001) This is the most popular preconditioning. Cooling of CC by 10°C lower than the ambient thermal environment (Nagai et.al., 2003). II.NO PRE-CONDITIONING: Power to the Heater and Cooling Liquid Flow is Turned on Simultaneously (Kurwitz & Best, 1997) INTRODUCTION : LHP START-UP PRE-HISTORY

6. 6 LHP LABORATORY SETUP IN CSA LOOP HEAT PIPE: Transferred from Russia (TAIS); Mounted and Equipped in CSA; Extended Ambient & Vacuum Environment Test Program is Started. Liquid Line Evaporator Condenser Vapor Line Vacuum Chamber Sink Plate Data Acquisition 6035A DC Power Supply Personal Work Station RC311 Cooler Thermocouples

7. 7 LHP LABORATORY SETUP IN CSA -Fixed layout of device: horizontal, -Fixed pre-conditioning: the condenser is cooled for 90 minutes before start-up attempt, -No start-up heaters and no pre-heating. -Fixed layout of device: horizontal, -Fixed pre-conditioning: the condenser is cooled for 90 minutes before start-up attempt, -No start-up heaters and no pre-heating. THERMOCOUPLES PLACEMENT TEST CONDITIONS -Fixed thermal environment: the ambient temperature 24°C, LHP was thermally insulated, -Fixed sink temperature at 13°C, -Fixed thermal environment: the ambient temperature 24°C, LHP was thermally insulated, -Fixed sink temperature at 13°C,

8. 8 TWO MAIN SUBJECTS of LHP TEST PROGRAM: Steady State Performance Despite recent intensive investigations on performances of LHPs, there are still many unexplained behavior in Start-up & Transient Performance

9. 9 TEST RESULTS: STEADY STATE PERFORMANCE 1.Is capable to transfer up to 400 W 2.Has High Thermal Conductance (~110 W/K) 3. Has Var Conductance Region in TVAC ( & Amb) 4. Has T-Hysteresis in TVAC Tests 5. Operates Very Reliable in Steady State Mode THE INVESTIGATED LHP:

10. 10 TEST RESULTS: TRANSIENT PERFORMANCE Two patterns of LHP operation during periodical LHP heating (45 W, 20 min cycling) Start-ups of LHP with periodical heating (23W, Cycle Intervals = 2,3,4,7,10,15 min) Start-up depends on many parameters & especially on the pre-history & pre-test conditions. Start-ups 35 o C 65 o C!

11. 11 TEST RESULTS: TRANSIENT PERFORMANCE Pattern No.2 of LHP operation during periodical heating. 74 W, 10, 20 min cycling. Two patterns of LHP operation during periodical LHP heating. 45 W, 10 min cycling 30 o C 65 o C!

12. 12 TEST RESULTS: TRANSIENT PERFORMANCE Example of Unsuccessful Start-up (Condenser was heated on 7 o C before the start) 40-hours Experiment with LHP periodical heating. Q= 59 W,  = 40 s to 180 min cycling 75 o C!!!

13. 13 CONCLUSIONS Tests of LHP Steady State and Transient (cycling heating by 23, 33, 45, 59, 74 & 92W with  intervals from 36 s to 2 h) Performance in VAC for COND T= 13 o C and horizontal orientation of the LHP were made. The LHP Variable Conductance region in VAC environment is condi- tioned by EV core heat leak. (In ambient conditions this phenomenon is governed by heat exchange with ambient air). Two main start-up temperature patterns were identified. Reliable start-ups at Q above turning point ~60 W were observed; below this Q start-up is not-yet predictable. For the reliable start-up and transient operation of LHP below the turning Q special pre-conditioning techniques and devices are recommended. The analysis of current available techniques was presented. Time Intervals (periods) & Regimes of the Transient Operation of LHP (payload electronics) are very important parameters for The LHP based Thermal Control System choice for Space applications.

14. 14 What is Next? Statistical Analysis and Verification of the Database (Additional Experimentation with other LHP devices and other initial and operational conditions) Determination of Start-up Regularities and Dependences on pre-history, environment conditions, etc. Steady-State, Start-up & Transient LHP Models and Computer Simulation Thermal Modeling Tools Development Development of not only recommendations, BUT techniques, methods and devices for reliable LHP Start- up Initiation and LHP Transient Operation.

15. 15 MODELING IN ECOSIMPRO HEAT STORAGE  HEAT REGENERATION WAVE  ADSORBER (heat source)  HEAT PUMP (solid-liquid)  HEAT PUMP (solid-solid)  HEAT PUMP (s-s) with HP TCS (?) CC Double LHP Evaporator Non-return Valves Adsorber No.2 Condenser Evaporator Throttling Valve Cooler Adsorber No.1

16. 16 MODELING IN ECOSIMPRO HEAT STORAGE  HEAT REGENERATION WAVE  ADSORBER (heat source)  HEAT PUMP (solid-liquid)  HEAT PUMP (solid-solid)  HEAT PUMP (s-s) with HP TCS (?)