Flow and heat transfer measurements inside a heated multiple rotating cavity with axial throughflow

This thesis discusses experimental results of measurement of heat transfer and velocity flow in a heated multiple cavity test rig with axial throughflow. Of particular interest are the internal cylindrical cavities formed by adjacent discs and the interaction of these with a central axial throughflow of cooling air. Tests were carried out for a range of non-dimensional parameters representative of gas-turbine high pressure compressor internal air system flows (ReΦ up to 5x106 and Rez up to 2x105). One configuration of the test rig was tested in the course of the reported study (Build 3) and test data from a previous rig configuration (Build 2) were processed, analysed and compared with the tested data. The most significant difference between the two builds of test rig was the size of the annular gap between the (non-rotating) shaft and the disc bores. Build 3 had a wider annular gap ratio, dh/b=0.164, while Build 2 featured a gap ratio of dh/b=0.092. Heat transfer data were obtained from thermocouples and a conduction analysis. Heat transfer results show differences between the versions of the rig, with the higher Nusselt number values in Build 3 attributed to the wider annular gap allowing more of the throughflow to penetrate into the cavity compared to Build 2. An attempt is made to correlate the average disc Nusselt numbers and this indicates the existence of different regimes. A two-component Laser Doppler Anemometry system was used on both rigs to measure cavity axial and tangential velocity components. Optical access in Build 3 also allowed for measurement of radial velocities. The axial and radial velocities inside the cavities are virtually zero. The size of the annular gap between disc bore and shaft has a significant effect on the radial distribution of tangential velocity. An analysis of the frequency spectrum obtained from the tangential velocity measurements shows evidence of periodicity in the flow consistent with the current understanding of the flow structure in a heated rotating cavity with axial throughflow

The effect of hydraulic diameter on flow boiling within single rectangular microchannels and comparison of heat sink configuration of a single and multiple microchannels

Phase change heat transfer within microchannels is considered one of the most promising cooling methods for the efficient cooling of high-performance electronic devices. However, there are still fundamental parameters, such as the effect of channel hydraulic diameter Dh, whose effects on fluid flow and heat transfer characteristics are not clearly defined yet. The objective of the present work is to numerically investigate the first transient flow boiling characteristics from the bubble inception up to the first stages of the flow boiling regime development, in rectangular microchannels of varying hydraulic diameters, utilising an enhanced custom VOF-based solver. The solver accounts for conjugate heat transfer effects, implemented in OpenFOAM and validated in the literature through experimental results and analytical solutions. The numerical study was conducted through two different sets of simulations. In the first set, flow boiling characteristics in four single microchannels of Dh = 50, 100, 150, and 200 μm with constant channel aspect ratio of 0.5 and length of 2.4 mm were examined. Due to the different Dh, the applied heat and mass flux values varied between 20 to 200 kW⁄m2 and 150 to 2400 kg⁄m2s, respectively. The results of the two-phase simulations were compared with the corresponding initial single-phase stage of the simulations, and an increase of up to 37.4% on the global Nu number Nuglob was revealed. In the second set of simulations, the effectiveness of having microchannel evaporators of single versus multiple parallel microchannels was investigated by performing and comparing simulations of a single rectangular microchannel with Dh of 200 μm and four-parallel rectangular microchannels, each having a hydraulic diameter Dh of 50 μm. By comparing the local time-averaged thermal resistance along the channels, it is found that the parallel microchannels configuration resulted in a 23.3% decrease in the average thermal resistance RRl compared to the corresponding single-phase simulation stage, while the flow boiling process reduced the RRl by only 5.4% for the single microchannel case. As for the developed flow regimes, churn and slug flow dominated, whereas liquid film evaporation and, for some cases, contact line evaporation were the main contributing flow boiling mechanisms

Performance of an Environmentally Friendly Alternative Fluid in a Loop Heat Pipe-Based Battery Thermal Management System

The present investigation aims to devise a thermal management system (TMS) for electric vehicles able to improve on limitations like charging time and all-electric range, together with the safety and environmental impact of the chosen thermal medium. A research gap is identified, as focus is often on addressing system thermal performance without considering that the thermal medium must not only provide suitable performances, but also must not add risks to both passengers and the environment. Thus, this work proposes an innovative cooling system including graphite sheets and a Loop Heat Pipe, filled with Novec™ 649 as working fluid, due to its exceptional environmental properties (GWP = 1 − ODP = 0) and safety features (non-flammable, non-toxic, dielectric). A three-cell module experimental demonstrator was built to compare temperatures when the proposed TMS is run with Novec™ 649 and ethanol. Results of testing over a bespoke fast charge driving cycle show that Novec™ 649 gave a faster start-up and a slightly higher maximum temperature (0.7 °C), meaning that the gains in safety and lower environmental impact brought by Novec™ 649 came without lowering the thermal performance. Finally, the TMS was tested under three different fast charge conditions (1C, 2C, 3C), obtaining maximum temperatures of 28.4 °C, 36.3 °C and 46.4 °C, respectively

A Modular Rack for Shared Thermo-Fluid Dynamics Experiments in Reduced Gravity Environment

Abstract Parabolic flights represent an important tool for short space-related experiments under reduced gravity conditions. During the ballistic flight manoeuvres, the investigators have the possibility to operate their experiments, in a laboratory-like environment, where the level of gravity subjected to the experiments repetitively in a series of periods of reduced gravity, preceded and followed by periods of hypergravity. Aboard large aircraft, the duration of this phases varies from approximately 20 s for a 0g flight up to up to 32 s for a Martian g level. A parabolic flight rack able to host experiments concerning thermo-fluid dynamics, has been designed, realized and qualified during the ESA 66th Parabolic Flight Campaign. This microgravity research platform, is the first UK facility available for such investigations, providing a data acquisition system, cooling system and heating system compliant with Novespace requirements

Infrared measurements of fluid temperature in a polymeric Pulsating Heat Pipe

Pulsating heat pipes are two-phase passive heat transfer devices partially filled with a working fluid in saturation conditions. During operation, supplying heat to one end of the system (named evaporator) results in a local increase in temperature and pressure, which drives the fluid through a transport section (named adiabatic section) towards the cooled, opposite end (named condenser) for effective heat dissipation. The local thermo-fluid dynamic state of the working fluid is sometimes assessed by means of non-intrusive techniques, such as infrared thermography. In this case, the radiative properties of the systems in the infrared spectrum must be known a priori. Nevertheless, since pulsating heat pipes may be manufactured with different materials, wall thicknesses and channel geometries, the radiative properties of the walls and the confined flow are not always known or assessable by means of the available literature. Hence, the work proposes to design a straightforward calibration procedure for quantitative infrared fluid temperature measurements in a polymeric pulsating heat pipe charged with FC-72 and having unknown radiative properties. The emissivity and transmissivity of the walls and confined fluid are estimated with good accuracy. The results will allow repeatable and reliable fluid temperature measurements in future experimentations on the mentioned device

A numerical investigation of the solid surface material influence on flow boiling within microchannels

Flow boiling within microchannel heat sinks constitute a promising solution for the cooling of high-performance electronic devices, dissipating high values of heat flux. Yet still, due to the complexity of flow boiling in small scales, the effect of important parameters is not clearly defined. In the present study a numerical investigation on the effect of solid surface thermophysical properties on flow boiling heat transfer characteristics within micro- channels, is conducted. For the proposed investigation an enhanced, custom VOF-based numerical model that has been developed in OpenFOAM is used. The utilised computational domain consists of a top rectangular fluid domain in contact with a bottom rectangular solid domain. The solid domain is heated at its bottom boundary by the application of a constant heat flux. In total five different solid surface materials were examined, focusing on the first transient stages of the confined two-phase flow development. The findings indicate that the investigated effect has a significant influence on the resulting two-phase regimes and in the associated heat transfer char- acteristics. High thermal conductivity materials such as silver, aluminium and copper exhibited the highest values of the time-averaged heat transfer coefficient with more than 35% increase compared to the single-phase stage of the simulations, whereas the brass and silver channels resulted in a lower increase of<30%. The flow boiling process in the brass and silver channels, was characterised by frequent bubble break-ups and a lower total vapour fraction values within the channels. The rest of the examined material cases were characterised by thicker liquid films and higher values of total vapour fraction. Finally, a new correlation for the global Nusselt number is proposed that takes into consideration the thermophysical properties of the solid domain

The effect of surface wettability on flow boiling characteristics within microchannels

The process of flow boiling within micro-passages plays a very important role in many industrial appli- cations. However, there is still a lack of understanding of the effect of an important controlling param- eter: surface wettability. In this paper, an advanced numerical investigation on the effect of wettability characteristics on single and multiple bubble growth during saturated flow boiling conditions within a microchannel is performed. The 3D numerical simulations are conducted with the open-source Computa- tional Fluid Dynamics (CFD) toolbox OpenFOAM, utilising a custom user-enhanced Volume Of Fluid (VOF) solver. The proposed solver enhancements involve an appropriate treatment for spurious velocities damp- ening, an improved dynamic contact angle treatment, as well as the implementation of a phase-change model in the fluid domain also accounting for Conjugate Heat Transfer (CHT) with the solid domain. In total, three sets of simulations of hydrophilic and hydrophobic surfaces with constant heat and mass flux were performed. In the first set, a single bubble seed was patched close to the inlet of the microchan- nel and the Heat Transfer Coefficient (HTC) along the channel interface was measured until the nose of the bubble reaches the outlet. The bubble growth and transport process within the channel were anal- ysed, with a minor effect of the wettability characteristics on the HTC observed. In the second set of simulations, multiple recurring nucleation events at the same position were simulated; observing that in such more realistic cases the effect of wettability in the HTC was more profound. Finally, simulations with multiple nucleation sites and recurring nucleation events were conducted to analyse cases closer to reality. These results show indeed that surface wettability plays a significant role on the HTC, with the hydrophilic and hydrophobic cases performing approximately 43.9% and 17.8% higher respectively, com- pared to the single-phase reference simulations. Additionally, it is found that the dominant heat transfer mechanisms for the hydrophilic and hydrophobic surface are liquid film evaporation and contact line evaporation, respectively, and that for the proposed simulation parameters liquid film evaporation can be considered as a more efficient heat transfer mechanism compared to contact line evaporation