EXPERIMENTAL AND COMPUTATIONAL INVESTIGATION OF THERMAL MANAGEMENT IN FLOW BOILING
Creators: name: Lee, Jeongmin
Publisher: name: Purdue University Graduate School
Year: 2022
Subjects: Experimental methods in fluid flow, heat and mass transfer
Computational methods in fluid flow, heat and mass transfer (incl. computational fluid dynamics)

description: The present study investigates the capability of computational fluid dynamics (CFD) extensively to predict hydrodynamics and heat transfer characteristics of FC-72 flow boiling in a 2.5-mm ´ 5.0-mm rectangular channel and experimentally explores system instabilities: <em>density wave oscillation</em> (DWO), <em>pressure drop oscillation</em> (PDO) and <em>parallel channel instability</em> (PCI) in a micro-channel heat sink containing 38 parallel channels having a hydraulic diameter of 316-μm. The computational method performs transient analysis to model the entire flow field and bubble behavior for subcooled flow boiling in a rectangular channel heated on two opposite walls at high heat flux conditions of about 40% – 80% of <em>critical heat flux</em> (CHF). The 3D CFD solver is constructed in ANSYS Fluent in which the <em>volume of fluid</em> (VOF) model is combined with a <em>shear stress transport</em> (SST) <em>k</em>-<em>ω</em> turbulent model, a surface tension model, and interfacial phase change model, along with a model for effects of shear-lift and bubble collision dispersion to overcome a fundamental weakness in modeling multiphase flows. Detailed information about bubble distribution in the vicinity of the heated surface, thermal conduction inside the heating wall, local heat fluxes passing through the solid-fluid interface, and velocity and temperature profiles, which are not easily observed or measured by experiments, is carefully evaluated. The simulation results are compared to experimental data to validate the solver’s ability to predict the flow configuration with single/double-side heating. The added momentum by shear-lift is shown to govern primarily the dynamic behavior of tiny bubbles stuck on the heated bottom wall and therefore has a more significant impact on both heat transfer and heated wall temperature. By including bubble collision dispersion force, coalescence of densely packed bubbles in the bulk region is significantly inhibited, with more giant bubbles even incurring additional breakup into smaller bubbles and culminating in far less vapor accumulation along the top wall. Including these momentums is shown to yield better agreement with local interfacial behavior along the channel, overall flow pattern, and heat transfer parameters (wall temperature and heat transfer coefficient) observed and measured in experiments. The computational approach is also shown to be highly effective at predicting local phenomena (velocity and temperature profiles) not easily determined through experiments. Different flow regimes predicted along the heated length exhibit a number of dominant mechanisms, including bubble nucleation, bubble growth, coalescence, vapor blankets, interfacial waviness, and residual liquid sub-layer, all of which agree well with the experiment. Vapor velocity is shown to increase appreciably along the heated length because of increased void fraction, while liquid velocity experiences large fluctuations. Non-equilibrium effects are accentuated with increasing mass velocity, contributing minor deviations of fluid temperature from simulations compared to those predicted by the analytical method. Predicted wall temperature is reasonably uniform in the middle of the heated length but increases in the entrance region due to sensible heat transfer in the subcooled liquid and decreases toward the exit, primarily because of flow acceleration resulting from increased void fraction. When it comes to analyzing heat transfer mechanisms at extremely high heat flux via CFD, predicted flow pattern, bubble behavior, and heat transfer parameters (such as wall temperature excursion and thermal energy concentration) clearly represent phenomena of premature CHF, which take place slightly earlier than actual operating conditions. But, despite these slight differences, the present computational work does demonstrate the ability to effectively predict the severe degradation in heat transfer performance commonly encountered at heat fluxes nearing CHF. Much of the published literature addressing flow instabilities in thermal management systems employing micro-channel modules are focused on instability characteristics of the module alone, and far fewer studies have aimed at understanding the relationship between these characteristics and compressive volume in the flow loop external to the module. From a practical point of view, developers of micro-channel thermal management systems for many modern applications are in pursuit of practical remedies that would significantly mitigate instabilities and their impact on cooling performance. Experiments are executed using FC-72 as a working fluid with a wide range of mass velocities and a reasonably constant inlet subcooling of ~15°C. The flow instabilities are reflected in pressure fluctuations detected mainly in the heat sink’s upstream plenum. Both inlet pressure and pressure drop signals are analyzed in pursuit of amplitude and frequency characteristics for different mass velocities and over a range of heat fluxes. The current experimental study also examines the effects of compressible volume location in a closed pump-driven flow loop designed to deliver FC-72 to a micro-channel test module having 38 channels with 315-μm hydraulic diameter. Three accumulator locations are investigated: upstream of the test module, downstream of the test module, and between the condenser and pump. Both high-frequency temporal parameter data and high-speed video records are analyzed for ranges of mass velocity and heat flux, with inlet subcooling held constant at ~15°C. PDO is shown to dominate when the accumulator is situated upstream, whereas PCI is dominant for the other two locations. Appreciable confinement of bubbles in individual channels is shown to promote rapid axial bubble growth. The study shows significant variations in the amount of vapor generated and dominant flow patterns among channels, a clear manifestation of PCI, especially for low mass velocities and high heat fluxes. It is also shown effects of the heat sink’s instabilities are felt in other components of the flow loop. The parametric trends for PCI are investigated with the aid of three different types of stability maps which show different abilities at demarcating stable and unstable operations. PDO shows severe pressure oscillations across the micro-channel heat sink, with rapid bubble growth and confinement, elongated bubble expansion in both directions, flow stagnation, and flow reversal (including vapor backflow to the inlet plenum) constituting the principal sequence of events characterizing the instability. Spectral analysis of pressure signals is performed using Fast Fourier Transform, which shows PDO extending the inlet pressure fluctuations with the same dominant frequency to other upstream flow loop components, with higher amplitudes closer to the pump exit. From a practical system operation point of view, throttling the flow upstream of the heat sink eliminates PDO but renders PCI dominant, and placing the accumulator in the liquid flow segment of the loop between the condenser and pump ensures the most stable operation.