Liquid Return Line Research Articles

Mechanical-capillary-driven two-phase loop: Feedback control for thin-film evaporation and capillary limit enhancement

• Feedback control can transition boiling modes in hybrid two-phase loop. • Capillary pumping head was used for feedback control as control variable. • Pump flow rate and evaporator liquid pressure were varied to extend capillary mode. • Pumping power consumption was reduced by feedback control of pump flow rate. • Feedback control was robust under transient heat inputs. Capillary-driven thin-film evaporation in the evaporator was modulated using feedback control of the mechanical pump flow rate and capillary pumping head in a mechanical-capillary-driven two-phase loop. The shift from cold-start (forced-convection dominant condition) to post cold-start (boiling dominant condition) was signified by a sudden drop in the evaporator thermal resistance at relatively low heat inputs. In the post cold-start, positive capillary pressure head resulted in another important transition of boiling conditions in the evaporator transitioning from mechanically-driven flooded boiling (flooded mode) to the capillary-driven thin-film evaporation (capillary mode) showing a small but continuous decrease in the thermal resistance. The feedback control significantly expanded the range of the capillary mode by 285% and reduced the mechanical pumping power consumption by 48% compared to a constant flow case. In this study using a monolayer wick in the evaporator, the highest capillary limit was measured to be 227.2 W/cm 2 with a thermal resistance of 0.190 K-cm 2 /W for the highest pump flow rate of 7.5 g/s and the highest flow restriction in the liquid return line from the evaporator.

Numerical and experimental investigations of the micro-channel flat loop heat pipe (MCFLHP) heat recovery system for data centre cooling and heat recovery

This paper represented a dedicated study of the effects of coolant water flow on the performance of the micro-channel flat loop heat pipe (MCFLHP) heat recovery system using both theoretical and experimental methods. The MCFLHP heat recovery system is novel in the micro-channel flat plate evaporator of the loop heat pipe attached tightly to the heat generator of the IT equipment and the micro-channel flat plate condenser of the loop heat pipe installed outside the data cabinet, which realises the long distance heat removal from the inside of the data cabinet to the outside and reduces the energy consumptions in the data centre. This system adopted the separate evaporation and condensation structures, which were connected into a complete loop through vapour transmission line and liquid return line, and employed the heat exchange part and chiller part as the heat recovery devices. Taking into account the heat balances occurring in different parts of the system, a computer model was developed to predict the dynamic performance of the MCFLHP heat recovery system 8 at different coolant water flow rates when other working conditions were unchanged. Experiments were also conducted to validate results obtained from the simulation. Through comparison between the testing and modelling results, the model has been validated to be able to give a reasonable accuracy for predicting the performance of the MCFLHP heat recovery system. It was found that the heat recovery efficiency increased firstly and then decreased with the increase of the coolant water flow rate, and 300 L/h was the most optimal coolant water flow rate for the proposed system. The instantaneous heat recovery efficiency of the MCFLHP heat recovery system was maintained within 81–94% under the optimal coolant water flow rate when the system was running stably, and the average efficiency was at 84.06%.

An Ultra-low Vibration Helium Reliquefier for Dual-Helmet Magnetoencephalography (MEG)

An ultra-low vibration helium reliquefier has been developed that can be integrated into an innovative magnetoencephalography (MEG) system with dual helmets for adult and pediatric use. The compact reliquefier establishes a closed-cycle helium loop for the MEG dewar allowing for uninterrupted, maintenance-free operation. The reliquefier, employing a Cryomech 4 K pulse tube cryocooler (model PT420), provides a liquefaction rate of 21 L/day from room temperature helium gas. Several vibration reduction measures have been applied to the system. The reliquefier incorporates a ~1.5 m long, horizontal, flexible liquid return line allowing it to be mounted outside of the magnetically shielded room (MSR). This arrangement greatly reduces vibrational, acoustic, and magnetic interference. In the prototype tests, this setup reached noise levels of ~18 fT/ÖHz without the use of noise cancellation software. The spontaneous brain activity signal showed nearly identical signal quality with the reliquefier turned on and off.

In−situ sludge reduction and carbon reuse in an anoxic/oxic process coupled with hydrocyclone breakage

The long−term performance of an anoxic−oxic−hydrocyclone (AOH) process with an in−situ hydrocyclone treatment unit in the mixed liquid return line for sludge reduction and carbon reuse has been observed, in comparison with a conventional anoxic−oxic (AO) process. Three parallel side−stream systems, including one AOH25 system with a 25−mm hydrocyclone, one AOH35 system with a 35−mm hydrocyclone and one AO system, were built and fed with real wastewater for a comparative study in a wastewater treatment plant. The results demonstrate that the hydrocyclone in the AOH process was able to break macro−flocs into smaller flocs. And the desorption of the extracellular polymeric substance from return activated sludge (AS) leaded to an average increase of 62.97% and 36.36% in SCOD in the AOH25 and AOH35 system, respectively. In addition, shear forces, centrifugal forces of revolution and flocs’ rotation in the hydrocyclone were proposed to be the main influence mechanism of hydrocyclone treatment on AS properties. Compared with the AO process, the SCOD concentration in the effluent of the AOH processes presented a decrease of 12.0 mg/L and the TN was reduced by 21.50% owing to the released carbon sources reuse. Moreover, the sludge production was reduced by 36.81% and 35.92% in the AOH25 and AOH35 process, respectively. By contrast, the AOH25 system was better than the AOH35 system.

Loop Heat Pipe for Spacecraft Thermal Control, Part 2: Ambient Condition Tests

Detailed results are presented of two ground-based tests carried out in September 1999 under ambient conditions (room temperature air and atmospheric conditions) to determine the thermal characteristics of loop heat pipes (LHP). One test (test 1) was carried out to quantify the effects that heat leaks have on the operational characteristics of the loop. In test 1, at various evaporator primary input power levels, auxiliary heating was applied to simulate heat leaks into the system. The intentional heat inputs representing heat leaks were located at the liquid return line and the compensation chamber. A second test (test 2) was conducted to verify the feasibility of temperature control of the LHP. Test 2 is motivated by the observation that it is desirable to control the temperature of an LHP to set the operating temperature for the system to which the LHP is attached

Loop Heat Pipe for Spacecraft Thermal Control, Part 1: Vacuum Chamber Tests

Detailed results of two ground-based tests to determine the thermal characteristics of loop heat pipes (LHP) with a view for application in spacecraft thermal control are presented. The tests were carried out in a thermal vacuum chamber. The first test involved one LHP and a deployable radiator, whereas the second test employed two LHPs and a deployable radiator. The data gathered from the thermal vacuum chamber tests have revealed the importance of correctly sizing the compensation chamber and fluid fill volume. The study has demonstrated the feasibility of a two LHP/deployable radiator system. The system responses to sudden large power decreases have been documented. The interaction between the two LHPs, the effects of the compensation chamber environment on performance, the effect of liquid return line routing on the radiator, and the effect of liquid return line heating are discussed. A key finding is the impact of heat leaks

Analysis of Boiling Flat-Plate Collectors

A detailed model for use with TRNSYS, capable of modelling a wide range of boiling collector types, was used to analyze boiling flat-plate collector systems. This model can account for a subcooled liquid entering the collector, heat losses in the vapor and the liquid return line, pressure drops due to friction in the collector and piping, and pressure drops due to the hydrostatic head of the fluid. The model has been used to determine the yearly performance of boiling flat-plate solar collector systems. A simplified approach was also developed which can be used with the f-Chart method to predict yearly performance of boiling flat-plate collector systems.