CHAPTER – 1INTRODUCTION1.1    BACKGROUNDHeat transfer (or heat) is thermal energy in transit due to the spatial temperature difference. Heat Exchanger is a device used to implement the process of heat exchange between two fluids that are at different temperatures and separated by a solid wall. The subject of enhanced heat transfer has developed to the stage that it is of serious interest for heat exchanger application. The refrigeration and automotive industries routinely use enhanced surfaces in their heat exchangers. The process industry is aggressively working to incorporate enhanced heat transfer surfaces in its heat exchangers.1.2    HEAT EXCHANGERA heat exchanger is a device that is used to transfer thermal energy between two or more fluids, between a solid surface and a fluid, or between solid particulates and a fluid, at different temperatures and in thermal contact. In heat exchangers, there are usually no external heat and work interactions. Typical applications involve heating or cooling of a fluid stream of concern and evaporation or condensation of single- or multicomponent fluid streams. In other applications, the objective may be to recover or reject heat or sterilize, pasteurize, fractionate, concentrate, crystallize, or control a process fluid. In a few heat exchangers, the fluids exchanging heat are in direct contact. In most heat exchangers, heat transfer between fluids takes place through a separating wall or into and out of a wall in a transient manner. In many heat exchangers, the fluids are separated by a heat transfer surface, and ideally, they do not mix or leak. Such heat exchangers are referred to as direct transfer type, or simply recuperators. In contrast, exchangers in which there is intermittent heat exchange between the hot and cold fluids — via thermal energy storage and release through the exchanger surface or matrix — are referred to as indirect transfer type, or simply regenerators. Such exchangers usually have fluid leakage from one fluid stream to the other, due to pressure differences and matrix rotation/valve switching. Common examples of heat exchangers are shell and tube exchangers, automobile radiators, condensers, evaporators, air preheaters, and cooling towers. If no phase change occurs in any of the fluids in the exchanger, it is sometimes referred to as a sensible heat exchanger. Figure – Classification of heat exchangersCombustion and the chemical reaction may take place within the exchanger, such as in boilers, fired heaters, and fluidized-bed exchangers. Mechanical devices may be used in some exchangers such as in scraped surface exchangers, agitated vessels, and stirred tank reactors. Heat transfer in the separating wall of a recuperator generally takes place by conduction. However, in a heat pipe heat exchanger, the heat pipe not only acts as a separating wall, but also facilitates the transfer of heat by condensation, evaporation, and conduction of the working fluid inside the heat pipe. In general, if the fluids are immiscible, the separating wall may be eliminated, and the interface between the fluids replaces a heat transfer surface, as in a direct-contact heat exchanger.A heat exchanger consists of heat transfer elements such as a core or matrix containing the heat transfer surface, and fluid distribution elements such as headers, manifolds, tanks, inlet and outlet nozzles or pipes, or seals. Usually, there are no moving parts in a heat exchanger; however, there are exceptions, such as a rotary regenerative exchanger (in which the matrix is mechanically driven to rotate at some design speed) or a scraped surface heat exchanger.The heat transfer surface is a surface of the exchanger core that is in direct contact with fluids and through which heat is transferred by conduction. That portion of the surface that is in direct contact with both the hot and cold fluids and transfers heat between them is referred to as the primary or direct surface. To increase the heat transfer area, appendages may be intimately connected to the primary surface to provide an extended, secondary, or indirect surface. These extended surface elements are referred to as fins. Thus, heat is conducted through the fin and convected (and/or radiated) from the fin (through the surface area) to the surrounding fluid or vice versa, depending on whether the fin is being cooled or heated. As a result, the addition of fins to the primary surface reduces the thermal resistance on that side and thereby increases the total heat transfer from the surface for the same temperature difference. Fins may form flow passages for the individual fluids but do not separate the two (or more) fluids of the exchanger. These secondary surfaces or fins may also be introduced primarily for structural strength purposes or to provide thorough mixing of a highly viscous liquid.Not only are heat exchangers often used in the process, power, petroleum, transportation, air-conditioning, refrigeration, cryogenic, heat recovery, alternative fuel, and manufacturing industries, they also serve as key components of many industrial products available in the marketplace. These heat exchangers can be classified in many different ways. We will classify them according to transfer processes, a number of fluids, and heat transfer mechanisms. Conventional heat exchangers are further classified according to construction type and flow arrangements. Another arbitrary classification can be made, based on the heat transfer surface area/volume ratio, into compact and non-compact heat exchangers. This classification is made because the type of equipment, fields of applications, and design techniques generally differ. All these classifications are summarized in Fig.1.3    THE ENHANCEMENT TECHNIQUESThey are broadly classified into three categories:1.    Active Techniques2.    Passive Techniques3.    Compound Techniques1.3.1    Active TechniquesIn these cases, external power is used to facilitate the desired flow modification and the concomitant improvement in the rate of heat transfer. Augmentation of heat transfer by this method can be achieved by following methods.1.    Mechanical aids involve stirring the fluid by mechanical means or rotating the surface. Mechanical surface scrapers, widely used for viscous liquids in the chemical process industry, can be applied to duct flow of gases. Equipment with rotating heat exchanger ducts is found in commercial practice.2.    Surface vibration at either low or high frequency has been used primarily to improve single-phase heat transfer. A piezoelectric device may be used to vibrate a surface and impinge small droplets onto a heated surface to promote “spray cooling.” 3.    Fluid vibration are primarily used in single phase flows and are considered to be perhaps the most practical type of vibration enhancement technique. The vibrations range from pulsations of about 1 Hz to ultrasound. Single-phase fluids are of primary concern. 4.    Electrostatic fields can be in the form of electric or magnetic fields or a combination of the two from dc or ac sources, which can be applied in heat exchange systems involving dielectric fluids. Depending on the application, it can also produce greater bulk mixing and induce forced convection or electromagnetic pumping to enhance heat transfer. 5.    Injection is utilized by supplying gas through a porous heat transfer surface to a flow of liquid or by injecting the same liquid upstream of the heat transfer section. The injected gas augments single-phase flow. Surface degassing of liquids may produce similar effects.6.    Jet impingement forces a single-phase fluid normally or obliquely toward the surface. Single or multiple jets may be used, and boiling is possible with liquids. 1.3.2    Passive Techniques1.    Extended surfaces: These are routinely employed in many heat exchangers. They provide effective heat transfer enlargement. The newer developments have led to modified finned surfaces that also tend to improve the heat transfer coefficients by disturbing the flow field in addition to increasing the surface area. In extended surfaces or fin, use of a plain fin may provide only area increase. However, formation of a special shape extended surface may also provide increased h. Current heat transfer enhancement efforts for gases are directed toward extended surfaces that provide a higher heat transfer coefficient than that of a plain fin design. These surfaces involve repeated formation and destruction of thin thermal boundary layers. Extended surfaces for liquids use much smaller fin heights than those used for gases. Shorter fin heights are used for liquids, because liquids typically have higher heat transfer coefficients than gases and other reason is lower operating pressure. Use of high fins with liquids would result in low fin efficiency, poor material utilization and higher operating pressure. 2.    Coated surfaces: These involve metallic or nonmetallic coating of the surface. Examples include a hydrophilic coating that promotes condensate drainage on evaporator fins, which reduces the wet air pressure drop, or a non-wetting coating, such as Teflon, to promote dropwise condensation.3.    Rough surfaces: They may be either integral to the base surface, or made by placing a “roughness” adjacent to the surface. Integral roughness is formed by machining, or “restructuring” the surface. For single-phase flow, the configuration is generally chosen to promote mixing in the boundary layer near the surface, rather than to increase the heat transfer surface area. The surface structuring forms artificial nucleation sites, which provide much higher performance than a plain surface. A wire coil insert is an example of a non-integral roughness. 4.    Displaced insert devices are devices inserted into the flow channel to improve energy transport at the heated surface indirectly. They are used with single- and two-phase flows. These inserts devices mix the main flow, in addition to that in the wall region. The wire coil insert is placed at the edge of the boundary layer, and is intended to promote mixing within the boundary layer, without significantly affecting the main flow. 5.    Swirl flow: These devices include a number of geometrical arrangements or tube inserts for forced flow that create rotating or secondary flow. Such devices include full-length twisted-tape inserts, or inlet vortex generators, and axial core inserts with a screw-type winding. There are also flow invertor or static mixer intended for laminar flows. They alternately swirl the flow in clockwise and counterclockwise directions. 6.    Coiled tubes: Mostly used more in compact heat exchangers. Secondary flow in the coiled tube produces higher single-phase coefficients and improvement in most boiling regimes. However, a quite small coil diameter is required to obtain moderate enhancement. 7.    Additives for liquids include solid particles or gas bubbles in single-phase flows and liquid trace additives for boiling systems. 1.3.3    Compound TechniquesWhen any two or more of these techniques are employed simultaneously to obtain enhancement in heat transfer that is greater than that produced by either of them when used individually, is termed as a compound enhancement. This technique involves complex design and hence has limited applications.1.4    CONCEPT OF NANOFLUIDDespite considerable previous research and development efforts on heat transfer enhancement major improvements have been constrained because of the low thermal conductivity of conventional heat transfer fluids. However, it is well known that at room temperature, metals in solid form have orders-of-magnitude higher thermal conductivities than those of fluids. For example, the thermal conductivity of copper at room temperature is about 700 times greater than that of water and about 3000 times greater than that of engine oil. The thermal conductivity of metallic liquids is much greater than that of non-metallic liquids. Therefore, the thermal conductivities of fluids that contain suspended solid metallic particles could be expected to be significantly higher than those of conventional heat transfer fluids.1.4.1    Importance of NanosizeThe basic concept of dispersing solids in fluids to enhance thermal conductivity is not new, and it can be traced back to Maxwell. Solid particles are added because as shown the figure X, they conduct heat much better than liquids. But the major problem is the rapid settling of these particles in the fluids. Other problems are abrasion and clogging, which seriously damage the application devices. Nanofluids have overcome these problems by forming stable suspensions and also by lasting for longer duration than millimetre or micrometre sized particles. The surface to volume ratio of nanoparticles is thousand times larger than that of micro particles. The high surface area of nanoparticles enhances the heat conduction of the nanofluids since heat transfer occurs on the surface of the nanoparticles. The number of atoms present on the surface of nanoparticles is very high as compared to interior. Thus, this unique property results in higher stability and higher thermal conductivity compared to other suspensions. Further, since nanoparticles are small, they may reduce erosion and clogging, thus also decreasing demand for pumping power. Figure – Thermal conductivity of materials Figure – Schematic representation of the multivariability of a nanofluid system. 1.4.2    Method of making nanoparticlesFabrication of nanoparticles can be classified into two broad categories: physical processes and chemical processes (Kimoto et al., 1963; Granqvist and Buhrman, 1976; Gleiter, 1989). Currently a number of methods exist for the manufacture of nanoparticles. Typical physical methods include inert-gas-condensation (IGC) technique developed by Granqvist and Buhrman (1976.), and mechanical grinding method. Chemical methods include chemical vapour deposition (CVD) method, chemical precipitation, micro emulsions, thermal spray, and spray pyrolysis. These nano-sized are most commonly produced in the form of powders. In powder form, nanoparticles are dispersed in aqueous or organic host liquids for specific applications.Dispersion of Nanoparticles in base fluidsStable suspensions of nanoparticles in conventional heat transfer fluids are produced by two methods: the two-step technique and the single-step technique. The two-step method first makes nanoparticles using one of the above-described nanoparticle processing techniques and then disperses them into the base fluids. The single-step direct evaporation method simultaneously makes and disperses the nanoparticles directly into the base fluids. In either case, a well-mixed and uniformly dispersed nanofluid is needed for successful reproduction of properties and interpretation of experimental data. Most of the nanofluids containing oxide nanoparticles and carbon nanotubes reported in the open literature are produced by the two-step process. If nanoparticles are produced in dry powder form, some agglomeration of individual nanoparticles may occur due to strong attractive van der Waals forces between the nanoparticles. This undesirable agglomeration is a key issue in all technology involving nanopowders. Making nanofluids using the two-step processes has remained a challenge because individual particles quickly agglomerates before dispersion and nanoparticle agglomerates settle out in the liquids. The well-dispersed, stable nanoparticle suspensions are produced by fully separating nanoparticle agglomerates into individual nanoparticles in a host liquid. In most nanofluids prepared by the two-step process, the agglomerates are not fully separated so nanoparticles are only partially dispersed. Although nanoparticles are ultrasonically dispersed in liquid using a bath or tip sonicator with intermittent sonication time to control overheating of nanofluids, this two- step preparation process produces significantly poor dispersion quality. Because dispersion quality is poor, the conductivity of nano fluid is very low. Therefore, the key to success in achieving significant enhancement in the thermal properties of nanofluids is to produce and suspend nearly mono dispersed or no-agglomerated nanoparticles in liquids.

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