Experimental Study and Mitigation of Pressure Drop Oscillation Using Active Control

2021 ◽  
Author(s):  
Qi Jin ◽  
John T. Wen ◽  
Shankar Narayanan
Keyword(s):  
Experimental Study ◽  
Pressure Drop ◽  
Active Control ◽  
Heat Load ◽  
Flow Instability ◽  
Flow Boiling ◽  
Flow Instabilities ◽  
System Response ◽  
System Parameters ◽  
Heat Loads

Abstract Flow boiling in microchannel evaporators is widely recognized and promising for its compact structure, lower coolant usage, high heat transfer coefficient, ability to provide higher heat fluxes, and better temperature uniformity than single-phase liquid cooling. However, critical heat flux, local dry-outs, and flow instabilities can be significant roadblocks for practical implementation. Flow instabilities, like pressure drop oscillation, could lead to non-uniform wall temperature distribution, flow reversal, and local dryout, which can be detrimental to system performance. We conducted an experimental study of a vapor compression cycle incorporating a microchannel evaporator to investigate the role of evaporator design and various system parameters on the overall performance. These parameters include the expansion valve setting, the accumulator heat load, and the evaporator heat load. While the evaporator design, the testbed, and system parameters affect the system response in unique ways, flow instability can be explained based on the overall pressure drop occurring in the system and how it varies as a function of these factors. Based on the understanding gained from this experimental study, a dynamic control strategy was developed to stabilize the system facing transient heat loads. The system can successfully address transient evaporator heat loads with feedforward control, which would otherwise lead to pressure drop oscillation. We believe this study can be helpful in further development of active control techniques to achieve multiple objectives of maintaining fixed evaporator temperature, allowing higher cooling rates, avoiding CHF, and suppressing flow instabilities, even in the presence of transient heat loads.

Experimental Study of Flow Patterns, Pressure Drop and Flow Instabilities in Parallel Rectangular Minichannels

2004 ◽  
Author(s):  
Prabhu Balasubramanian ◽  
Satish G. Kandlikar
Keyword(s):  
Pressure Drop ◽  
High Speed ◽  
Flow Instability ◽  
Flow Boiling ◽  
Pressure Fluctuations ◽  
Flow Patterns ◽  
Flow Instabilities ◽  
Stable Operation ◽  
Phase System ◽  
High Heat Flux

The use of phase change heat transfer in parallel minichannels and microchannels is one of the solutions proposed for cooling high heat flux systems. The increase in pressure drop in a two phase system is one of the problems, that need to be studied in detail before proceeding to any design phase. The pressure drop fluctuations in a network of parallel channels connected by a common head need to be addressed for stable operation of flow boiling systems. The current work focuses on studying the pressure-drop fluctuations and flow instabilities in a set of six parallel rectangular minichannels, each with 333 μm hydraulic diameter. Demonized and degassed water was used for all the experiments. Pressure fluctuations are recorded and signal analysis is performed to find the dominant frequencies and their amplitudes. These pressure fluctuations are then mapped to their corresponding flow patterns observed using a high speed camera. The results help us to relate pressure fluctuations to different flow characteristics, and their effect on flow instability.

Flow Boiling Heat Transfer and Two-Phase Flow Instability of Nanofluids in a Minichannel

2015 ◽  
Vol 137 (5) ◽  
Author(s):  
Leyuan Yu ◽  
Aritra Sur ◽  
Dong Liu
Keyword(s):  
Heat Transfer ◽  
Boiling Heat Transfer ◽  
Two Phase Flow ◽  
Flow Instability ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Flow Instabilities ◽  
Phase Flow ◽  
Two Phase ◽  
Flow Boiling Heat Transfer

Single-phase convective heat transfer of nanofluids has been studied extensively, and different degrees of enhancement were observed over the base fluids, whereas there is still debate on the improvement in overall thermal performance when both heat transfer and hydrodynamic characteristics are considered. Meanwhile, very few studies have been devoted to investigating two-phase heat transfer of nanofluids, and it remains inconclusive whether the same pessimistic outlook should be expected. In this work, an experimental study of forced convective flow boiling and two-phase flow was conducted for Al2O3–water nanofluids through a minichannel. General flow boiling heat transfer characteristics were measured, and the effects of nanofluids on the onset of nucleate boiling (ONB) were studied. Two-phase flow instabilities were also explored with an emphasis on the transition boundaries of onset of flow instabilities (OFI). It was found that the presence of nanoparticles delays ONB and suppresses OFI, and the extent is correlated to the nanoparticle volume concentration. These effects were attributed to the changes in available nucleation sites and surface wettability as well as thinning of thermal boundary layers in nanofluid flow. Additionally, it was observed that the pressure-drop type flow instability prevails in two-phase flow of nanofluids, but with reduced amplitude in pressure, temperature, and mass flux oscillations.

Experimental Investigation on Flow Instability in a Single Vertical Microtube

2015 ◽  
Author(s):  
Qian You ◽  
Ibrahim Hassan ◽  
Lyes Kadem
Keyword(s):  
Heat Flux ◽  
Pressure Drop ◽  
Mass Flux ◽  
Flow Instability ◽  
Flow Boiling ◽  
Flow Direction ◽  
Density Wave ◽  
Mass Fluxes ◽  
Flow Oscillations ◽  
Flow Directions

The experiments are conducted to study the flow boiling instability in a single microtube with 0.889 mm hydraulic diameter in vertical upward and downward flow directions (VU and VD). The subcooled dielectric liquid FC-72 is driven at mass fluxes varying from 700 to 1400 kg/m2·s, and the heat flux uniformly applied on the microtube surface is up to 9.6 W/cm2. The onsets of flow oscillations (OFIs) in both flow directions are observed. Their oscillation types and characteristics are presented as well. The effects of mass flux and heat flux on flow instability in vertical flow directions are discussed. The results show that as the mass flux increases, the OFI occurrence is postponed, and the compounded oscillation types (Ledinegg, pressure drop and density wave oscillations) turn to pressure drop type dominant. At low mass fluxes, the OFI appears earlier in VD than in VU due to the buoyancy force impeded the bubble discharging. As the mass flux increases, the OFI appearance in VD is close to the ones in VU and its flow oscillations tend to be re-stabilized. After OFIs appeared at a given mass flux, with more heat flux added, the density wave oscillation type in VU becomes more active. However, at a constant mass flux, as the heat flux increases, the flow instability in VD becomes “stable” which may be due to the rapid flow pattern change, and this kind of “stable” is not expected because the local dryout may accompany. Hence, the microtube with vertical upward flow direction (VU) performs better from flow boiling instability point of view.

Active Flow Instability Control for Transient Two-Phase Electronics Cooling

2010 ◽  
Author(s):  
Tie Jun Zhang ◽  
Yoav Peles ◽  
John T. Wen ◽  
Michael K. Jensen
Keyword(s):  
Active Control ◽  
Flow Instability ◽  
Cooling System ◽  
Flow Instabilities ◽  
Future Research ◽  
Two Phase ◽  
Control Schemes ◽  
Boiling Flow ◽  
Flow Oscillations ◽  
Two Phase Cooling

Because of increasing power densities, refrigeration systems are being explored for two-phase cooling of ultra high power electronic components. Flow instabilities are potential problems in any two-phase refrigeration cooling system especially in transient applications. Oscillatory two-phase flow in a boiling channel can trigger transition to the critical heat flux (CHF). Active control methods can help better dynamic thermal management of electronic systems, even though transient two-phase boiling flow mechanisms are complicated. This paper presents a framework for the transient analysis and active control of pressure-drop flow instabilities under varying imposed heat loads. The first part of the paper is to study the external effects on boiling flow characteristics and the boiling oscillatory flow responses to transient heat load changes. Then based on the theoretical analysis of boiling flow oscillations, a set of active control schemes are developed and studied to suppress flow oscillations and, therefore, to increase the CHF. With the available control devices (i.e., inlet valve and supply pump), different active control schemes are studied to improve the transient two-phase cooling performance. Finally, a discussion is included to address potential future research.

Flow Boiling of Carbon Dioxide in Horizontal Mini-Channel and Pattern Dynamics Approach to Study Flow Pattern

2009 ◽  
Author(s):  
Mamoru Ozawa
Keyword(s):  
Carbon Dioxide ◽  
High Pressure ◽  
Pressure Drop ◽  
Flow Pattern ◽  
Flow Instability ◽  
Flow Boiling ◽  
Flow Patterns ◽  
Bond Number ◽  
Pattern Dynamics ◽  
Mini Channels

This paper provides a brief review on experimental and numerical investigations of flow patterns, pressure drop, and heat transfer including critical heat flux (CHF) of flow boiling carbon-dioxide (CO2) at high pressure in mini-channels ranging 0.5mm to 3.0mm in diameter. The flow patterns of CO2 at high pressure with small density difference between vapor and liquid and low surface tension show a slightly different structure from so far observed in mini-channels with air and water. The phase mal-distribution, similar to conventional tubes, in the cross-section becomes rather significant beyond the critical Bond number, which leads to the intermittent dryout at the upper wall of the tube. So far proposed flow pattern transition criteria are ineffective there, and newly developed discrete bubble model demonstrates its high potential in predicting flow patterns. Conventional homogeneous flow model is still available in predicting pressure drop. Based on this fact, flow instability problems, which significantly affect CHF, is discussed focusing on high-pressure CO2 flow.

scholarly journals Effects of Dissolved Air on Subcooled Flow Boiling of a Dielectric Coolant in a Microchannel Heat Sink

2005 ◽  
Author(s):  
Tailian Chen ◽  
Suresh V. Garimella
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Pressure Drop ◽  
Flow Instability ◽  
Flow Boiling ◽  
Microchannel Heat Sink ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Boiling Process ◽  
Dissolved Air

The effects of dissolved air in the dielectric liquid FC-77 on flow boiling in a microchannel heat sink containing 10 parallel channels, each 500 μm wide and 2.5 mm deep, were experimentally investigated. Experiments were conducted before and after degassing, at three flow rates in the range of 30 to 50 ml/min. The dissolved air resulted in a significant reduction in wall temperature at which bubbles were first observed in the microchannels. Analysis of the results suggests that the bubbles observed initially in the undegassed liquid were most likely air bubbles. Once the boiling process is initiated, the wall temperature continues to increase for the undegassed liquid, while it remains relatively unchanged in the case of the degassed liquid. Prior to the inception of boiling in the degassed liquid, the heat transfer coefficients with the undegassed liquid were 300–500% higher than for degassed liquid, depending on the flow rate. The heat transfer coefficients for both cases reach similar values at high heat fluxes (over 120 kW/m2) once the boiling process with the degassed liquid was well established. The boiling process induced a significant increase in pressure drop relative to single-phase flow; the pressure drop for undegassed liquid was measured to be higher than for degassed liquid once the boiling process became well established in both cases. Flow instabilities were induced by the boiling process, and the magnitude of the instability was quantified using the standard deviation of the measured pressure drop at a given heat flux. It was found that the magnitude of flow instability increased with increasing heat flux in both the undegassed and degassed liquids, with greater flow instability noted in the undegassed liquid.

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