Experimental study of electrohydrodynamic pumping through conduction phenomenon

This paper provides a summary of recent experimental work in electrohydrodynamic (EHD) conduction pumping of dielectric liquid, done by the authors and others in the Multi Scale Heat Transfer Laboratory at WPI. The primary objective of all experiments was to gain a fundamental understanding of interaction of flow fields and electrical fields in the presence and absence of phase change (liquid-vapor) and presence and absence of gravity. This fundamental research has led to development of potential applications in electrically-driven heat transfer enhancement. Meso- and micro-scale EHD conduction pumps were studied as mechanisms for flow generation in single- and two-phase heat transport systems. In addition, EHD conduction pumping was studied as a technique to augment pool boiling and liquid film boiling heat transfer. An analytical model was developed to characterize the influence of an electrical field on pool boiling critical heat flux. Most recently, two experiments were conducted on board a parabolic flight to study EHD conduction-driven liquid film boiling and micro-scale, single-phase flow in microgravity.

Electrohydrodynamic (EHD) conduction pump is a new concept which has been recently explored. Net pumping is achieved by properly utilizing the heterocharge layers present in the vicinity of the electrodes. This paper presents an innovative electrode design which provides pressure head on the order of 600 Pa per an electrode pair operated at 20 kV with less than 0.08 W of electric power. An EHD conduction pump consisting of six pairs was designed and installed in the liquid passage of a monogroove heat pipe. The EHD conduction pump drastically increased the heat transport capacity of the heat pipe. Furthermore, the EHD conduction pump provided immediate recovery from the dryout condition.

Heat pipes

A heat pipe can efficiently transfer heat through it liquid-vapor phase change mechanism. Among different heat pipes, loop heat pipe (LHP) and pulsating heat pipe (PHP) are known to be capable of high heat flux/high load heat transfer. In this article, LHP and PHP heat transfer systems are combined to achieve passive, reliable and long-distance heat transfer for successful thermal management of modern avionic systems. Aiming at this development goal, a 2m long LHP is designed to transport heat from the avionics chassis to the remote heat rejection site. To reduce saturation pressure and ensure structural safety at higher operating temperature, water is used as the operating fluid of LHP. Within the avionic chassis, conduction heat transfer is enhanced by sandwiching and solder-bonding a PHP between two PCBs (printed circuit boards). Each PHP/PCB assembly is 20cm long and 12.5cm wide, with electrical heaters mounted to mimic electronic heat dissipation. Heat transfer demonstration of the LHP and PHP combo system is conducted in a lab environment with input power varying from 100 to 400W. For all three PHP/PCB assemblies set in the avionic chassis, the maximum heat source temperature is less than 130°C, even when input power is twice as much as the state of the art. The enhanced heat transfer consumes less power, increases system reliability, and enables avionic system operation in harsher environments.

Heat pipes are well-known as simple and effective heat transport devices, utilizing two-phase flow and the capillary phenomenon to remove heat. However, the generation of capillary pressure requires a wicking structure and the overall heat transport capacity of the heat pipe is generally limited by the amount of capillary pressure generation that the wicking structure can achieve. Therefore, to increase the heat transport capacity, the capillary phenomenon must be either augmented or replaced by some other pumping technique. Electro-hydrodynamic (EHD) conduction pumping has been demonstrated as an effective method for pumping liquid films by using DC electric fields and a dielectric working fluid. Beyond increased pumping capacity, EHD conduction pumping offers other advantages over capillary pumping, such as active control of the pumping capacity via the intensity of the applied electric field. This experimental study demonstrates the prospects of a macro-scale two-phase heat transport device that is driven by EHD conduction pumping. Various liquid film thicknesses are considered. In each case, the performance of the EHD-driven heat transport device at various electric field intensities is compared to the capabilities of the same device under gravity alone. The effect of tilt on the device is also considered.

Comparative study of a novel liquid–vapour separator incorporated gravitational loop heat pipe against the conventional gravitational straight and loop heat pipes – Part II: Experimental testing and simulation model validation

From Day 325, 2004 (launch) through Day 101, 2010, there is a total of 3 anomalies on the Swift Instrument Module (IM) Thermal Control System (TCS). Two are concerned with the Burst Alert Telescope (BAT) loop heat pipe (LHP) heater circuits. On Day 90, 2010, the primary heater circuit of the BAT LHP #0 compensation chamber (CC) failed to power on by the Power Converter Box (PCB). It caused a rapid decrease in the CC temperature and activated its secondary heater circuit which had a 2oC lower heater controller set point. The 2oC difference in temperature set points between the LHP #0 CC and LHP #1 CC led to thermal instability in the entire LHP thermal system and Blocks. After the LHP #0 CC secondary heater controller set point was increased to that of LHP #1, thermal stability has been restored and the temperatures of the LHP thermal system and Blocks are nominal. On Day 97, 2005, a failure in temperature controllability of the BAT LHP #1 CC primary heater circuit caused temperature excursions that led to a LHP #1 shut down. It had no impact on Gamma Ray Burst (GRB) detection because LHP #0 was nominal. After LHP #1 was started up and its primary heat controller was disabled on Day 98, both LHPs had been nominal until Day 90, 2010. On Day 337, 2004, the X-Ray Telescope (XRT) thermo-electric cooler (TEC) power supply (PS) suffered a single point failure. The charge-coupled device (CCD) has been cooled by the radiator passively to -50oC or colder most of the time. The CCD temperature meets the main objective of pinpointing GRB afterglow positions. With these 3 anomalies overcome, the IM TCS is nominal after more than 5 years in flight. Redundancy in thermal control hardware and electronics boxes that supply power to thermal control hardware is highly recommended to prevent a mission single point failure.

<div class="htmlview paragraph">NASA's Earth Observing System (EOS) Program will place a series of unmanned polar-orbiting spacecraft in low-earth orbit to support a variety of scientific and Earth-observing missions. The EOS AM spacecraft has a requirement for an advanced heat transportation system for thermal energy. A Capillary Pumped Heat Transportation System (CPHTS) using Capillary Pumped Loop (CPL) technology was selected to accommodate the instruments requiring advanced heat transport. The CPHTS is comprised of several major subassemblies: CPL Coldplate for instrument heat transfer interface; Heat Pipe Heat Exchanger (HPHX) used to exchange energy from the CPL loop vapor and the radiator mounted heat pipes; Constant Conductance Heat Pipes (CCHP) used to interface with the HPHX and distribute the over the radiator area; Non-Condensible Gas Trap (NCG Trap) used to remove non-condensible gases from the subcooled CPL liquid returning from the HPHX; and Reservoir used to control liquid inventory and system temperature setpoint. This paper describes the hardware design and development for the Coldplate's, HPHX's, CCHP's, and NCG Traps.</div>

Analysis of the thermal resistance of a loop heat pipe based on the P-T diagram of the working fluid operating cycle.

Studies on the processing of nickel base porous wicks for capillary pumped loop for thermal management of spacecrafts

A loop heat pipe (LHP) is an efficient method of conserving energy in data center cooling applications. In scenarios where the installation is constrained by height or distance limitations, pump driving is needed. This paper examines the performance changes induced by a gas pump both experimentally and theoretically. An adjustable, oil-free linear compressor is utilized as a gas pump. The evaporator is a finned-tube heat exchanger and the condenser is a water-cooled plate heat exchanger. When the filling ratio of the working fluid is insufficient, employing a gas pump can enhance the heat transfer performance. However, when the filling ratio of the working fluid is sufficient, while the gas pump can increase the flowrate of the working fluid, the heat transfer rate (HTR) does not change significantly. In fact, it may reduce the energy efficiency ratio of the heat pipe. Infrared thermography has proven to be an efficient tool for estimating the area ratio of different zones within the evaporator, which is crucial for the output regulation of the compressor. The area ratio of the two-phase zone is nearly linear to the HTR. Through the establishment of a physical model of a gas-pump-driven loop heat pipe (GPLHP), the impacts of the LHP size and gas pump operation on the heat transfer performance are analyzed. It is found that the gas pump can extend the application range of the LHP, although it has a minimal impact on the maximum HTR. How to select a gas pump for an LHP is discussed.

Thermal boundary layer depletion in minichannels by electrohydrodynamic conduction pumping

<div class="htmlview paragraph">In this paper, experimental work performed on a breadboard Loop Heat Pipe (LHP) is presented. The test article was built by DCI for the Geoscience Laser Altimeter System (GLAS) instrument on the ICESat spacecraft. The thermal system requirements of GLAS have shown that ammonia cannot be used as the working fluid in this LHP because GLAS radiators could cool to well below the freezing point of ammonia. As a result, propylene was proposed as an alternative LHP working fluid since it has a lower freezing point than ammonia. Both working fluids were tested in the same LHP following a similar test plan in ambient conditions. The thermal performance characteristics of ammonia and propylene LHP's were then compared. In general, the propylene LHP required slightly less startup superheat and less control heater power than the ammonia LHP. The thermal conductance values for the propylene LHP were also lower than the ammonia LHP. Later, the propylene LHP was tested in a thermal vacuum chamber. These tests demonstrated that propylene could meet the GLAS thermal design requirements. Design guidelines were proposed for the next flight-like Development Model (DM) LHP for thermal control of the GLAS instrument.</div>

Numerical and experimental study on the vapor-liquid distribution of loop heat pipe with varying density of working fluid

The ongoing development of modern electronic systems leads to smaller, more powerful devices that are expected to operate in complex environments. Due to this, advanced thermal management technologies are required to meet the growing demand, especially in space where two-phase thermal systems are limited by the absence of gravity. Electrohydrodynamic (EHD) and dielectrophoretic (DEP) forces can be used to sustain stable liquid film flow boiling in the absence of gravity, which is otherwise impractical due to the lack of a required buoyancy force to initiate bubble departure. EHD is a phenomenon that is represented by the interaction between electric fields and fluid flow. The DEP force is characterized by its ability to act on liquid/vapor interfaces due to a high gradient of electrical permittivity. This study investigates the heat transfer characteristics of EHD conduction pumping driven liquid film flow boiling coupled with DEP vapor extraction during a microgravity parabolic flight and on the ground. The results of this study show that EHD and DEP raise the critical heat flux, lower heater surface temperature, and successfully sustain boiling in both microgravity and on the ground with low power consumption. Additionally, the heat transfer data captured in terrestrial, microgravity, and 1.8 g conditions compare well, indicating that combining these mechanisms can provide thermal enhancement independent of gravity. This study provides fundamental understanding of electrically driven liquid film flow boiling in the presence of phase change, paving the way toward developing next-generation heat transport devices for space and terrestrial applications.