1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer 1.6 Applications of Multiphase Heat Transfer
Selected applications of multiphase systems in these technologies are reviewed in this section. 1.6.1 Energy Systems Including Fuel Cells and Combustors Thermal Energy Storage
Figure 1.17 Schematic of latent heat thermal energy storage system. Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 1 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer The major barrier to more widespread use of solar energy is its periodic feature, i.e., it is available only during daytime, and so a heat storage device is needed to store energy and release it for use at night. The latent heat thermal energy storage system, which utilizes phase-change materials (PCMs) to absorb and release heat, is widely used for this purpose. The PCM in the thermal energy storage system is molten when the system absorbs heat, and it solidifies when the system releases heat. The advantages of the latent heat thermal energy storage system are that: a large amount of heat can be absorbed and released at a constant temperature the size of the latent heat thermal energy system is considerably smaller than its counterpart using sensible heat thermal energy storage. Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 2 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Power and Refrigeration Cycles The condenser is a major component in power plants as well as in air conditioning units and refrigerators. The condenser converts exhaust steam/refrigerant vapor into liquid and by rejecting heat to the ambient environment. Fig. 1.18 shows a schematic diagram of a typical water-cooled condenser for a modern power plant. The cooling water flows inside the tubes and the exhaust steam condenses on the outside surfaces of the tubes. To improve the efficiency of the steam power plant, it is desirable to increase the average heat-addition temperature. One practical way to increase the temperature of feedwater entering the boiler is to incorporate a feedwater heater, which uses steam extracted from various points of the steam turbine to heat the feedwater.
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Figure 1.18 Water-cooled condenser (Lock, 1994).
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Figure 1.19 Feedwater heater concept.
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Figure 1.20 Boiler (Lock, 1994). Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 6 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Phase Change during Combustion Combustion is a chemical reaction process between a fuel (which can be solid, liquid or gas) and an oxidant to produce high-temperature gases, which can be used to generate steam in a boiler, drive a gas turbine, or melt metals in a metallurgical process. Except in cases where gaseous fuels are used, the combustion process always involves phase changes.
Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 7 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer When solid fuel, such as coal, is used in the combustor, combustion occurs on the surface of the solid fuel. In order to increase the contact area between the coal and oxidant, the coal is ground into fluidized particles (very small particles that can flow with the oxidant gas) that are consumed during the combustion. Combustion of solid fuel involves gas-solid two-phase flow interaction between solid particles diffusion of oxidant near the particle surfaces conduction heat transfer in the solid particle convective heat transfer in the gas as well as chemical reactions on the particle surface that consume the solid particles Since the densities of solid fuel and oxidant are significantly different, resulting flow patterns are usually not homogeneous because the solid particles and the oxidant possess different velocities.
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1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Fan
Compressors
Combustor Fan Stream
Primary Stream
Turbines
Figure 1.21 Components and main flows of the turbofan engine (courtesy of GE-P&W Engine Alliance, LLC).
Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 9 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Diffuser Case Fuel Nozzle
Liner Figure 1.22 Schematic of typical turbofan combustor.
Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 10 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Fuel Cells A fuel cell is an electrochemical energy device that converts the chemical energy in the fuel directly into electrical energy. It is becoming increasingly attractive alternatives to other conversion technologies, from small-scale passive devices like batteries to large-scale thermodynamic cycle engines. Unlike conventional power devices, i.e., steam turbines, gas turbines and internal combustion engines, which are based on certain thermal cycles, the maximum efficiency of fuel cells is not limited by the Carnot cycle principle. A fuel cell generally functions as follows: 1. electrons are released from the oxidation of fuel at the anode, 2. protons (or ions) pass through a layer of electrolyte, 3. the electrons are required for reduction of an oxidant at the cathode. The desired output is the largest flow of electrons possible over the highest electric potential. Although other oxidants such as the halogens have been used where high efficiency is critical, oxygen is the standard because of its availability in the atmosphere. Fuel cells typically use hydrogen, carbon monoxide or hydrocarbon fuels (i.e., methane, methanol). The hydrogen and carbon monoxide fuels may be the products of catalytically processed hydrocarbons.
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In the first case, the oxidant at the cathode combines with electrons, which tend to circumvent the electrolyte, and becomes anions which travel through the electrolyte to the anode At the anode, the anions give up their electrons and combine with hydrogen to form water. The water, depleted fuel, and products are exhausted from the anode surface, and the depleted oxidant and products are exhausted from the cathode surface. In the second case, where the electrolyte conducts cations, the hydrogen containing fuel is decomposed electrochemically, giving up electrons and leaving hydrogen cations to travel through the electrolyte. Upon reaching the cathode the cations combine electrochemically with the oxidant and electrons, which tend to circumvent the electrolyte to form water.
Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 12 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Oxidant (e.g., Oxygen, Air….) Reactant Products (e.g., H2O, CO2 ….) & Heat
Cathode
Load Electrolyte e- Anode
Fuel (e.g., H2, CH4, CO, CH3OH….) Figure 1.23 Fuel cell schematic.
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There are several types of fuel cells, and they belong to either of the two cases just described. Types of anion-conducting electrolyte fuel cells are: Alkaline Fuel Cells – for example, those using potassium hydroxide molten carbonate that operates at about 650C, and Solid Oxide Fuel Cells that operates to 1000C. Cation-conducting electrolyte fuel cells include Phosphoric Acid Fuel Cells Polymer Electrolyte Membrane Fuel Cells.
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The fuel cell stack consists of repeated interleaved structure of MEAs, GDLs and bipolar plates. It is evident that flow channels are an essential component for flow distribution in many PEMFC deigns. The flow channels in a PEM fuel cell are typically on the order of a 1 mm hydraulic diameter, which falls into the range of minichannels (with hydraulic diameters from 200 μm to 3 mm). As shown in Fig. 1.24, one channel wall is porous (gas diffusion layer); mass transfer occurs on this wall along its length. Hydrogen is consumed on the anode side along the main flow dimension in minichannels. Oxygen from air is introduced on cathode side to form water at catalyst sites at the cathode, which is transported into the minichannels through the gas diffusion layer and then is eventually removed from the cell by the gas flow and gravity if so oriented.
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Bipolar Plate Bipolar Plate e-
H2O H2O
+ e- H
H+
H2 O2 H2 O2 Figure 1.24 Basic construction of a typical PEM fuel cell stack (Faghri and Guo, 2005)
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Gas Purification Hydrogen (H2)
CO2 + H2
Reformer Fresh Air and Fuel Cells Catalytic Burner
Cooling Pump Compressor/ Expander
Vaporizer
Cooler
Cooling Cooling Air Out Air In Water Methanol Tank Tank Water produced by fuel cells
Figure 1.25 Methanol powered fuel cell system
Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 17 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer 1.6.2 Food and Biological Material Processing Food Freezing and Thawing
Food freezing is solidification of multicomponent liquid in porous media To preserve the structure of the food cells the temperature of the freezer needs to below enough to achieve rapid freezing Thawing Convection thawing Microwave heating
Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 18 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer 1.6.2 Food and Biological Material Processing Food Freezing and Thawing
Cryopreservation of Biological Materials Uses liquid nitrogen to deep-freeze multicomponent substances at (-196°C) Oocytes, embryos, tissues, organs Very challenging because both freezing and thawing can cause severe damage to cells
Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 19 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer 1.6.3 Laser Assisted Manufacturing Selective Laser Sintering (SLS) of Metal Powder SLS is an emerging technology of Solid Freeform Fabrication (SFF) in which 3-D parts can be built from CAD data. SLS involves fabrication of near-full-density objects from powdered material via layer-by-layer sintering or melting induced by a directed laser beam (generally CO2 or YAG). Thin (100 - 250 mm) powder layers are laser-scanned to fuse a densified two-dimensional slice to an underlying solid piece, which consists of a series of stacked and fused two- dimensional slices. After laser scanning, the part bin is lowered by one layer thickness, fresh powder layer is spread, and the scanning process is repeated. Loose powder is removed after the part is extracted from its bin.
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Figure 1.26 Selective laser sintering (Marcus, et al., 1993)
Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 21 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Laser Machining Laser machining involves removal of material through vaporization of the portion of the workpiece that interacts with the laser beam. The mechanism of vaporization of the workpiece material is different for metal and ceramic materials. Figure 1.27 shows the physical model of laser drilling on a metal substrate. A laser beam is directed toward a solid target material at an initial temperature of Ti, which is below the melting point of the metal. The laser-material interaction can be divided into three stages. During the first stage, the temperature of the solid remains below the melting point so that no melting or vaporization occurs. The solid absorbs thermal energy and its temperature increases with time. When the highest temperature of the solid – located at the center of the laser beam – reaches the target material’s melting point, continued laser beam irradiation results in melting of the target material; at this point the process enters its second stage.
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Figure 1.27 Laser Drilling physical model. (Ganesh et al., 1997).
Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 23 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer SALD and SALDVI are based on the concept of building functional structures through deposition of solid materials from gas precursors by a laser beam in an environmentally-controlled chamber. Both techniques utilize Laser Chemical Vapor Deposition, which can be based on reactions initiated pyrolytically, photolytically or a combination of both, to deposit film to desired location or to join powder particles together. While the SALD technique uses precursors to directly create free-standing parts or to join together simple shapes to create parts with higher complexity, the SALDVI uses gas precursors and powder particles to build three-dimensional parts.
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(a) SALD (b) SALDVI
Figure 1.28 Schematic of SALD and SALDVI (Jakubenas et al., 1997)
Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 25 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer This is similar to other SFF techniques, such as Selective Laser Sintering (SLS). The advantages of SALDVI over SALD include a) uninfiltrated powder provides support for producing overhangs b) confining the deposition to thin powder layers provides dimensional control in the direction of growth, and c) it is possible to tailor local chemistry and micro structures. The manufacturing process using SALDVI is very similar to that using SLS because both of them fabricate objects from powdered material via a layer-by-layer process induced by a directed laser beam. The only difference is that the powder particles in SALDVI are bonded together by LCVD, which occurs on the surface of the powder particles, while binding of powder particles in SLS is accomplished through sintering or melting of powder particles.
Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 26 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer 1.6.4 Heat Pipes
Heat pipes are devices used to transfer heat via the processes of evaporation and condensation. Relative to highly-conductive materials like copper, they can be designed to move larger quantities of heat over longer distances through narrower spaces at lower temperature differentials.
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Figure 1.30 Principle of heat pipe Figure 1.29 Thermosyphon. with wick (Faghri, 1995).
Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 28 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer The wick heat pipe, on the other hand, can operate in any orientation because it uses a wick to distribute the liquid. The principle of wick heat pipe operation: Heat is applied to the evaporator section and is conducted through the wick and liquid. Liquid evaporates at its interface with vapor as it absorbs the applied heat. In the condenser section, the vapor releases heat to its cooler interface with liquid as it condenses. In the wick, the menisci are increasingly pronounced approaching the evaporator end due to the growing pressure drop required to draw the liquid through the increasing length of wick. There are additional contributions to pressure drop, such as friction of the vapor flow and adverse orientation against gravity or other acceleration sources. Subsequently, the vapor pressure drops as it flows from the evaporator to the condenser. The friction of the liquid flow through the wick causes the liquid pressure to drop from the condenser to the evaporator. If the heat pipe is to function, all pressure drop sources need to be balanced by the capillary pressure differential provided at the menisci in the capillary wick.
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Figure 1.31 Pulsating heat pipes: (a) unlooped, (b) looped (Shafii et al., 2001). Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 30 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer There is no countercurrent flow between the liquid and vapor. The entrainment limit in the conventional heat pipe does not have any effect on the capacity of heat transport by a PHP. With this simple structure, the PHP weighs less than a conventional heat pipe, and is an ideal candidate for space applications. Since the diameter of the PHP is very small, surface tension plays a greater role in the dynamics of PHP than does gravity force and enables successful operation in a microgravity environment; this feature makes PHPs even more attractive option for space application. Other applications of PHPs include thermal control of electrical and electronic devices and components, as well as cooling of thyristors, diodes and ceramic resistors.
Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 31 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer 1.6.5 Electronics Cooling Condensation in Miniature Tubes
Figure 1.32 Condensation in miniature tube.
Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 32 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer The term “miniature tube” is referred to as a tube with a diameter of 3.5 mm or less. Unlike a conventionally-sized passage, in which surface tension effects are of limited importance, surface tension in a miniature tube can have a significant impact on the overall hydrodynamics and in particular on the thin films. Capillary blocking can occur in forced convective condensation in miniature tubes as a result of surface tension, in which case the liquid blocks the tube cross-section at some distance from the condenser entrance (Fig. 1.32). As a result there is no contribution to heat removal by the part of the tube that is blocked by the liquid. Accurate prediction of the condition of capillary blocking and the length of the effective condensation length is crucial for the design of an electronic cooling system that involves condensation in miniature channels
Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 33 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Micro Heat Pipes
High local heat removal rates Convex but cusped cross-sections Hydraulic diameters from 10 to 500 μm Transfer heat only in axial direction Flat plate heat spreaders
Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 34 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Heat Sinks A heat sink is a device that absorbs heat generated by electronic components or chips. Among the different types of heat sinks, two-phase forced-convection cooling of high-heat-flux/high-power electronic devices is one of the most effective means of thermal management. This method becomes especially important due to the ongoing trend of miniaturization and the increase in power dissipation per unit surface area of modern electronic devices, which has already reached 300 W/cm². Various two-phase miniature heat sinks have been investigated and presented in the literature to meet this challenging demand. For example, presented flat miniature heat sinks with enhanced inner surfaces ranging from smooth to shallow and wide trapezoidal micro grooves, to deep and narrow rectangular grooves (Fig. 1.33). The heat transfer mechanism in the heat sink can be evaporation or/and boiling in the micro grooves. It is imperative to ensure that the heat flux applied to the heat sink does not exceed the critical heat flux (CHF), above which the temperature of the electronic device increases significantly, and may lead to failure of the heat sink.
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Figure 1.33 Flat miniature heat sinks (Hopkins et al., 1999) Transport Phenomena in Multiphase Systems by Amir Faghri & Yuwen Zhang 36 1.6 Applications of Multiphase Chapter 1: Introduction Heat Transfer Heat Spreaders In addition to the limitations on maximum chip temperature, there may be further constraints on the level of temperature uniformity in electronic components. The micro heat pipe is a very promising technology for achieving high local heat removal rates and uniform temperatures in computer chips. Micro heat pipe structures can be fabricated on the substrate surface of electronic chips using the same technology that forms the circuitry. These thermal structures can be an integral part of the electronic chip and remove heat directly from the area where the maximum dissipation occurs.
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Fig. 1.34 Heat spreader
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