LAMINAR SLUG FLOW - HEAT TRANSFER CHARACTERISTICS WITH CONSTANT HEAT FLUX BOUNDARY

The problem of elevated heat flux in modern electronics has led to the development of numerous liquid cooling devices which yield superior heat transfer coefficients over their air based counterparts. This study investigates the use of liquid/gas slug flows where a liquid coolant is segregated into discrete slugs, resulting in a segmented flow, and heat transfer rates are enhanced by an internal circulation within slugs. This circulation directs cooler fluid from the center of the slug towards the heated surface and elevates the temperature difference at the wall. An experimental facility is built to examine this problem in circular tube flow with a constant wall heat flux boundary condition. This was attained by Joule heating a thin walled stainless steel tube. Water was used as the coolant and air as the segregating phase. The flow rates of each were controlled using high precision syringe pumps and a slug producing mechanism was introduced for segmenting the flow into slugs of various lengths at any particular flow rate. Tube flows with Reynolds numbers in the range 10 to 1500 were examined ensuring a well ordered segmented flow throughout. Heat transfer performance was calculated by measuring the exterior temperature of the thin tube wall at various locations using an Infrared camera. Nusselt number results are presented for inverse Graetz numbers over four decades, which spans both the thermally developing and developed regions. The results show that Nu in the early thermally developing region are slightly inferior to single phase flows for heat transfer performance but become far superior at higher values of inverse Gr. Additionally, the slug length plays an important role in maximizing Nusselt number in the fully developed region as Nu plateaus at different levels for slugs of differing lengths. Overall, this paper provides a new body of experimental findings relating to segmented flow heat transfer in constant heat flux tubes without boiling.Put abstract text here. Copyright © 2009 by ASME

Heat transfer model for gas-liquid slug flows under constant flux

This paper investigates the mechanisms leading to enhanced heat and/or mass transfer rates in two-phase non-boiling slug flows. The problem is analyzed in a minichannel geometry subjected to a constant heat flux boundary. Local Nusselt numbers, obtained using Infrared thermography are analyzed in both entrance and fully developed flow regions. These novel measurements highlight the physics governing slug-flow heat transfer and results indicate that optimized slug geometries can yield up to an order of magnitude heat transfer enhancement. Finally, based on the physics identified, a heat transfer model is developed which is also applicable to similar mass transfer problems. © 2010 Elsevier Ltd. All rights reserved

Heat Transfer Enhancement Using Laminar Gas-Liquid Segmented Plug Flows

Heat transfer enhancement using segmented nonboiling gas-liquid flow is examined. Segmentation results in a two phase flow of liquid/gas having a constant liquid fraction; i.e., no phase change occurs. In this flow configuration, enhanced heat transfer occurs as a result of a shorter effective thermal length due to internal fluid circulation in the liquid plugs. A simple theory for laminar segmented flows is developed based on scaled Graetz theory and comparisons made with existing published data from the literature and new experimental data obtained in a companion study. The proposed model is valid for an isothermal tube wall provided that the axial residence time of the flow is such that dimensionless tube length L* <0.1. © 2011 American Society of Mechanical Engineers

Simple Models for Laminar Thermally Developing Slug Flow in Noncircular Ducts and Channels

Solutions to the classical Graetz slug flow problem (uniform velocity distribution) in noncircular ducts are examined. These solutions have applications where a constant uniform velocity distribution exists across a channel or duct. These are most often realized in the laminar flow of low Prandtl number liquids, such as liquid metals, and low Reynolds number flows through porous media. Expressions are developed for a number of applications using the asymptotic correlation method of Churchill and Usagi. These expressions vary depending on the definition used for the dimensionless heat transfer coefficient, in the case of constant wall temperature boundary condition (T), and the dimensionless wall temperature for the constant flux boundary conditions (H) and (H1). Finally, simple expressions are developed for predicting the thermal entrance length and fully developed flow Nu values for noncircular ducts. © 2010 American Society of Mechanical Engineers

Heat transfer enhancement using laminar gas-liquid segmented fluid streams

Heat transfer enhancement using segmented non-boiling gas-liquid streams is examined. Segmentation results in a two phase flow of liquid/gas having a constant liquid fraction, i.e. no phase change occurs. In this flow configuration, enhanced heat transfer occurs as a result of a shorter effective thermal length and internal fluid circulation in the liquid plugs. A simple theory for laminar segmented flows is developed based on Graetz theory and comparisons made with existing data from the literature and new data obtained in a companion study. Copyright © 2009 by ASME

LAMINAR SLUG FLOW - HEAT TRANSFER CHARACTERISTICS WITH CONSTANT HEAT FLUX BOUNDARY

The problem of elevated heat flux in modern electronics has led to the development of numerous liquid cooling devices which yield superior heat transfer coefficients over their air based counterparts. This study investigates the use of liquid/gas slug flows where a liquid coolant is segregated into discrete slugs, resulting in a segmented flow, and heat transfer rates are enhanced by an internal circulation within slugs. This circulation directs cooler fluid from the center of the slug towards the heated surface and elevates the temperature difference at the wall. An experimental facility is built to examine this problem in circular tube flow with a constant wall heat flux boundary condition. This was attained by Joule heating a thin walled stainless steel tube. Water was used as the coolant and air as the segregating phase. The flow rates of each were controlled using high precision syringe pumps and a slug producing mechanism was introduced for segmenting the flow into slugs of various lengths at any particular flow rate. Tube flows with Reynolds numbers in the range 10 to 1500 were examined ensuring a well ordered segmented flow throughout. Heat transfer performance was calculated by measuring the exterior temperature of the thin tube wall at various locations using an Infrared camera. Nusselt number results are presented for inverse Graetz numbers over four decades, which spans both the thermally developing and developed regions. The results show that Nu in the early thermally developing region are slightly inferior to single phase flows for heat transfer performance but become far superior at higher values of inverse Gr. Additionally, the slug length plays an important role in maximizing Nusselt number in the fully developed region as Nu plateaus at different levels for slugs of differing lengths. Overall, this paper provides a new body of experimental findings relating to segmented flow heat transfer in constant heat flux tubes without boiling.Put abstract text here. Copyright © 2009 by ASME

Heat transfer enhancement with laminar liquid-gas slug flows

This paper investigates a two-phase non-boiling slug flow regime for the purposes of enhancing heat transfer in microchannel heat sinks or compact heat exchangers. The primary focus is upon understanding the mechanisms leading to enhanced heat transfer and also the effect of utilizing different Prandtl number fluids. Experiments were conducted using Infrared thermography and results presented in terms of Nusselt number versus inverse Graetz parameter. These results spanned both the thermal entrance and fully developed flow regions. Nusselt numbers enhancements were observed throughout when data was reduced to account for void fraction. The maximum heat transfer rates observed were up to an order of magnitude greater than those in equivalent single phase flows. However, the extent of enhancement observed was strongly dependent on ratio of slug length to channel dimension with shorter liquid slugs providing optimum performance. It was also highlighted that the thermal entrance region for the slug flow regime analyzed was independent of flow Reynolds number and instead was characterized by the liquid slug length alone. This was verified through Nusselt number measurements over inverse Graetz number ranges from 10 -4 to 100 and slug length to channel diameter ratio from 1 to 32. The results obtained also highlight some interesting variations between the transitions from entrance to fully developed flow when using different Prandtl number fluids. Low Prandtl number fluids show a strong oscillation in heat transfer rates resulting from an internal circulation within liquid slugs however, these diminished significantly when the high Prandtl number fluids were employed. Overall, this study highlights the mechanisms which offer significantly heat transfer enhancements in heat exchange devices employing two-phase gas-liquid flows without boiling. ©2010 IEEE

A Novel Approach to Low Profile Heat Sink Design

This paper discusses the importance of developing cooling solutions for low profile devices. This is addressed with an experimental and theoretical study on forced convection cooling solution designs that could be implemented into such devices. Conventional finned and corresponding finless designs of equal exterior dimensions are considered for three different heat sink profiles ranging from 1 mm to 4 mm in combination with a commercially available radial blower. The results show that forced convection heat transfer rates can be enhanced by up to 55% using finless designs at low profiles with relatively small footprint areas. Overall, this paper provides optimization and geometry selection criteria, which are relevant to designers of low profile cooling solutions. © 2010 by ASME

Heat transfer in gas-liquid and liquid-liquid two phase plug flow systems

Liquid-gas and liquid-liquid two phase flows are examined. It has been recently documented that gas-liquid segmented flows offer a thermal enhancement advantage over single phase flows, since the plug flow (Taylor flow) regime produces internal circulations which enhance radial transport. Additional strategies in liquid-gas and liquid-liquid cooling are being pursued with liquid cooled heat sinks. Proper heat transfer and pressure drop modelling are required in order to maximize the benefit of segmented fluid streams. This paper will present an overview of recent strategies and models, along with new results obtained by the authors for mini-channel heat sinks. These new results highlight some operational issues which need further attention. ©2010 IEEE

AN EXPERIMENTAL AND THEORETICAL STUDY OF FINNED AND FINLESS HEAT SINKS FOR LOW PROFILE APPLICATIONS

This paper discusses the importance of developing cooling solutions for low profile devices. This is addressed with an experimental and theoretical study on forced convection cooling solution designs that could be implemented into such devices. Conventional finned and corresponding finless designs of equal exterior dimensions are considered for three different heat sink profiles ranging from 1mm to 4mm profile in combination with a commercially available radial blower. The results show that forced convection heat transfer rates can be enhanced by up to 55% using finless designs at low profiles with relatively small footprint areas. The advantages of both finned and finless geometries are presented along with the limitations of the customary finned heat sink design at low profile scales. The results also show large increases in heat transfer rates over that predicted which can be attained at the low profile scale based on geometry selection. Dimensionless comparisons are made between experimental results and combined hydrodynamic and thermally developing duct flow theory which is representative of the flow regime within both the finned and finless geometries. Overall, this paper provides optimization and geometry selection criteria which are relevant to designers of low profile cooling solutions. Copyright © 2009 by ASME