CHAPTER 2 LITERATURE SURVEY

Transcription

1 5 CHAPTER 2 LITERATURE SURVEY 2.1 INTRODUCTION The thermo physical properties required for calculation of convective heat transfer coefficient and Nusselt number are thermal conductivity, viscosity, specific heat and density. Properties of single phase fluids are well documented and are available in heat transfer data books. But on the other hand the thermo physical properties of two phase fluids are not much available and needs to be measured by conducting experiments. Different researchers have elaborated experimental procedure for measurement of nanofluid properties. Earlier studies reported that nanofluid properties vary with temperature and the particle volume concentration in the basefluid. The thermal conductivities of two phase nanofluids are comparatively higher than those of base fluids. Properties of nanofluids can be advantageously altered to make them suitable particularly in heat transfer applications. Conventional heat transfer fluids can be replaced by nanofluids due to many advantages offered by the nanofluids. Thermal conductivity is one of the important properties that influence heat transfer in nanofluids. Hence it is proposed to collect literature pertaining to the research work done on nanofluid properties and nanofluid heat transfer performance. The heat transfer

2 6 coefficient of nanofluids depends on fluid mass flow rate, type of flow that is whether the flow is laminar or turbulent and also on swirl in the flow, created by different inserts. Hence the literature collected is classified into five headings viz, literature on properties of nanofluids, literature on single phase fluids, literature on nanofluids in laminar and turbulent flow conditions and research papers on heat transfer augmentation using different types of inserts are collected, reviewed and presented as follows. 2.2 LITERATURE REVIEW ON NANOFLUID PROPERTIES A novel idea of two-phase fluid ie a liquid with nano particles present in it is conceived and this fluid mixture is expected to give high thermal conductivity. Nano particles of metals and metal oxides dispersed in any conventional heat transfer fluids show higher thermal conductivities when compared to the thermal conductivities of pure liquids. In the last 100 years, a number of theoretical and experimental studies were undertaken on the properties of liquid suspensions containing milli or micro sized particles. Touloukian and Ho (1970) have proved experimentally that at room temperature, the thermal conductivity of Cu is 700 times more than that of water and 3000 fold more than that of engine oil. Hamilton and Crosser (1962) and Wasp (1977) have developed a thermal conductivity models for two-phase mixture based on their theoretical study. Sohn and Chen (1984) investigated thermal conductivity property of solid-fluid mixture at low velocity. At higher

3 7 flow rate (higher peclet number), the thermal conductivity was observed to be increasing with increase in the shear rate. Masuda et al. (1993) studied the possibility of altering the properties of conventional heat transfer fluids by suspending submicron particles of water based Al203 and Ti02 and reported that the enhancement in the effective thermal conductivities are about 32% and 11%, respectively for the nanofluids of 4.3% volume concentration. Choi (1995) is the first researcher who worked on nano particles at the Argonne National Laboratory, USA. He demonstrated that nanofluids exhibit an increased thermal conductivity compared to the host fluid. Eastman et al. (1997) observed that oxide nanoparticles, such as Al2O3 and CuO have excellent dispersion properties in water, oil and ethylene glycol and form stable suspensions. Wang et al. (1999) employed a steady state parallel plate method to measure the effective thermal conductivity of nanofluids. They tested two types of nanoparticles, Al2O3 and CuO, dispersed in water, engine oil, and ethylene glycol. Experimental results indicated higher thermal conductivities in fluid mixture than those of the base fluids and the measured thermal conductivity values are higher for nanofluids and the mixture formula under predicted experimental thermal conductivity of the above nanofluids. Choi et al. (2001) noticed that engine oil of carbon nanotubes and with 1.0% volume concentration exhibited 160% increment in thermal conductivity. Das et al. (2003) employed temperature

4 8 oscillation technique to measure thermal conductivity of water based Al2O3 and CuO nanofluids at different temperatures and observed a 200% to 400% increase in the thermal conductivity of nanofluids in the temperature range of 21 0 C to 51 0 C. Xue and Xu (2005) developed an effective thermal conductivity model for CuO/water and CuO/Ethylene glycol nanofluids taking into account the thermal conductivity of the solid and liquid, their relative volume fraction, particle size and interfacial properties. Koo and Kleinstreuer (2005) studied conduction-convection heat transfer characteristics of water ethylene/cuo nanofluids in micro channels and developed new models for thermal conductivity and viscosity including the effect of viscous dissipation. Murshed et al. (2005) used spherical and cylindrical shaped TiO2 nanoparticles in water and measured the thermal conductivities applying hot wire method. Their results revealed that the thermal conductivity increased with increase in particle volume fraction. The particle size and shape also have a bearing on enhancement of thermal conductivity. The ph value and viscosity of the nanofluids are also characterized in their experimental work. Liu et al. (2006) produced Cu nanoparticles of around nm in diameter by chemical reduction method and nanofluid is prepared without adding a surfactant. In a 0.1% volume concentration Cu nanofluid, a 23.8% enhancement in the nanofluid thermal conductivity was reported by them. Wang et al (2006) measured

5 9 thermal conductivities of Carbon Nano Tubes (CNT) in water, CuO in water, SiO2 in water, and CuO in ethylene glycol by transient hot-wire method and reported 11.3% improvement in the thermal conductivity of water-based CNT nanofluids with 0.01% volume concentration. The measured thermal conductivity found to be relatively higher than the thermal conductivity calculated using Hamilton Crosser conductivity model. Beck et al. (2007) measured the thermal conductivity of ethylene glycol based alumina nanofluids in the temperature range of 298 to 411K using a transient hot wire method. Higher thermal conductivities were reported for all concentrations of nanofluids compared to the base liquid. Casquillas et al. (2007) conducted experiments on thermal conductivity properties of ethylene glycol based nanofluids of carbon nanotubes at droplet level and found that nanotubes concentration has a strong effect on thermal conductivity. Chen et al. (2007) studied and measured shear viscosity of ethylene glycol-titania nanofluids upto 8% percent on particle weight basis and concluded that the shear viscosity of the nanofluids is a strong function of particle concentration and nanofluid temperature. Honga et al. (2007) worked on nanofluids containing carbon nanotubes and Fe2O3 particles and brought to light that the thermal conductivity of nanofluids can be improved by applying external magnetic field. He reasoned that the Fe2O3 particles align in the form of chains under applied magnetic

6 10 field and help to connect the nanotubes, which results in enhanced thermal conductivity. Lee et al. (2008) estimated both thermal conductivities and viscosity properties of Al2O3 -water nanofluids and observed that both the properties are linearly increasing with increase in the nanoparticle concentration. Li et al. (2008) investigated the combined effects of PH variation and surfactant (Sodium Dodecyl Benzene Sulfonate) Cu nanofluids. They observed that the thermal conductivity of Cu/H2O nanofluids is more dependent on the weight fraction of nanoparticle, ph value of nanofluid and surfactant concentration. For a Cu nanoparticles of 0.01 % concentration with an optimal PH value and surfactant, highest thermal conductivity up to 10.7% was reported. Suitable surfactant and optimum PH value play a role to improve thermal conductivity of nanofluids for heat transfer applications. Mintsa et al. (2008) measured the effective thermal conductivity of water based alumina and copper oxide nanofluids. Their results have shown an increase in the effective thermal conductivity of nanofluids with an increase in particle volume concentration and with a decrease in particle size. It is also noticed that the relative increase in thermal conductivity was predominant at higher temperatures. Murshed et al. (2008) stated that thermal conductivity of nanofluids depends on factors like particle shape, size, interfacial layer, and temperature in addition to the particle volume fraction. Zhu et al. (2008) studied the dispersion behavior and thermal conductivity of

7 11 Al2O3/H2O Nanofluid by varying its ph values and Sodium Dodecyl Benzene Sulphonate concentration. The nanofluids exhibited better dispersion behavior when the surfactant is added in the Nanofluid. Lee et al. (2008) measured the effective viscosities and thermal conductivities of low concentration water-al2o3 nanofluids. The viscosity of nanofluids decreased with increase in the temperature. But the measured thermal conductivity, on the other hand increased linearly with increase in nanofluids concentration. Namburu et al. (2008) investigated the rheological property of copper oxide nanoparticles in ethylene glycol-water mixture base fluid by varying the nanoparticle concentrations from 0% to 6.12% in the temperature range from 35 0 C to 50 0 C. The nano fluid also exhibited Newtonian behavior in the concentration range tested. For a volume concentration of 6.12%, the viscosity of copper oxide nanofluid four fold higher than that of the base fluid at 35 0 C. 2.3 LITERATURE ON SINGLE PHASE HEAT TRANSFER FLUIDS Researchers working on the heat transfer characteristics of single phase fluids have brought out many new findings with regard to convective heat transfer coefficients and friction factor in different flow regimes ranging from laminar to turbulent conditions. Heat transfer is an important aspect to be studied especially in power plants, chemical industries, process plants, air conditioning and automotive industries. Conventional liquids such as distilled water, glycols and mineral oils are very often used as heat transfer fluids.

8 12 Blasius (1908) has developed a regression equation for prediction of friction factor for a turbulent flow in terms of the Reynolds number. Dittus Boelter(1930) studied fully developed single phase fluid, flowing in a circular plain tube and described the procedure for estimation of Nusselt number. Sider and Tate (1936), Shah (1975), Notter-Rouse (1976) developed a correlation for calculation of heat transfer coefficient of fluids flowing in a circular tube in laminar flow conditions. The researchers like Churchill and Usagi (1972) have developed a correlation for estimation of pressure drop and Nusselt number for a laminar flow in a plain circular tube. Gnielinski (1976) in his experimental study on single phase fluid in transition to turbulent flow conditions has developed a regression equation which predicts Nusselt number. Tam and Ghajar (2006) developed a regression equation for the estimation of Nusselt number for a fluid flow in the transition regime. 2.4 LITERATURE ON LAMINAR NANOFLUIDS HEAT TRANSFER With the advent of recent advances in the nanotechnology, nanomaterial synthesis, processing and characterization was made possible and production of both metallic and non-metallic particles in nano dimension with controlled structure is realized. In nanomaterials several parameters like shape, size and rapid interface alter the properties of nanoparticles which are quite different for the same materials in macroscopic dimension. The base liquids containing nanoparticles are known as Nanofluids. Choi (1995) for the first time

9 13 coined this word of Nanofluid. Many experimental studies have been taken up by different research groups to investigate the transport behavior of nanofluids. The phase change behavior of nanofluids was studied by Lee and Choi (1996), Pak and Cho (1999), Xuan and Roetzel (2000), Li and Xuan (2002), Das et al. (2003), Li et al (2003), Tsai et al(2003), You et al (2003), Khanafer and Vafai, (2003) and Vassallo et al. (2004). Lee and Choi (1999) and Xuan and Li (2000) have studied the convective heat transfer coefficient of a unspecified nanofluids in the laminar flow condition in a micro channel and reported a three fold increase in the heat dissipation rate when compared to the heat transfer rate of the base liquid. Xuan and Li (2003) investigated on heat transfer behavior of water based copper Nanofluids. In their experimental study, about 60% enhancement in the heat transfer coefficient of a 2 Volume % Cu nanofluids was reported at a particular Reynolds number. Yang et al. (2005) carried heat transfer performance study of graphite nanofluids in laminar flow regime and reported enhancement in the heat transfer coefficient nanofluids. The Nature of nanoparticles, particle concentration, thermal properties of base liquid and the temperature influence preparation of nanofluids for better heat transfer applications. Heris et al. (2006) in their experimental work on CuO/ water and Al2O3 / water nanofluids in laminar flow regime and subjected to constant wall temperature boundary condition stated that for both the

10 14 nanofluids the heat transfer rate increased with increase in the nanoparticle concentration and also with Peclet number. The reasons attributed to enhanced heat transfer are increased thermal conductivity, random and chaotic movements and fluctuations and interactions among nanoparticles. Heris et al. (2007) conducted an experimental study exclusively on Al2O3/water Nanofluid with nanopartcle concentration from 0.2 to 2.5% volume fraction in the laminar flow condition with constant wall temperature boundary condition and observed higher heat transfer coefficients with the nanofluids. It was concluded that in addition to increased thermal conductivity of nanoparticles, other parameters like dispersion and chaotic movement of nanoparticles, Brownian motion, particle migration, and particles interaction may augment heat transfer in the Nanofluids. 2.5 LITERATURE ON TURBULENT NANOFLUID HEAT TRANSFER Convective heat transfer enhancement is achieved by passive techniques either by changing the flow geometry or by increasing the thermal conductivity of liquids. Many theoretical and experimental works have been carried out on liquid suspensions containing solids particles of micro dimensions with the aim of increasing convective heat transfer coefficient since Maxwell s (1881) theoretical work on particle suspension. The use of course grained particles was dispensed with because of instability of suspensions and possible erosion of pipe material by course grained particles.

11 15 When compared to micro sized particles, the nano particles provide relatively larger surface area and facilitate formation of stable suspensions and the problems of clogging, sedimentation and erosion can be minimized to a greater extent. The use of nanofluids as working fluids in heat exchanger leads to miniaturization of its components. The progress in the nanofluids development is hindered even to day because of lack of agreement among various research findings, poor characterization of nanofluids, and lack of theoretical base to understand of the particle transport mechanism. Presence of nanopaticles in base liquid changes its heat transfer performance considerably. Appropriate theory needs to be developed to understand the mechanism as to how the nanofluids exhibit higher and favorable thermo physical properties, for commercial application of nanomaterials. In recent years a novel technique of using phase change particles was introduced by Kasza and Chen (1982), Pak and Cho (1989), Charunyakorn et al. (1991) for increasing the convective heat transfer coefficient in a turbulent flow. Choi et al. (1994) in their experimental work they used a phase-change materials (Tetradecane) of 0.4mm in size and with a surfactant (for dispersion and preventing clogging) and illustrated significant increase in the heat transfer coefficient in turbulent flow in a circular pipe. Pak and Cho (1999) conducted experiments to study the hydrodynamic and convective heat transfer behaviors of submicron particles of metal oxide in

12 16 turbulent flow under constant heat flux boundary condition. He et al. (2007) investigated the effects of particles concentrations, size, and the flow rate on heat transfer and flow characteristics of aqueous TiO2 nanofluids flowing in a straight vertical pipe considering both the laminar and turbulent flow regimes. Nguyen et al. (2007) studied the heat transfer behavior of water based Al2O3 nanofluids for cooling of microprocessors. Their experimental results clearly demonstrated a 40% enhancement in the convective heat transfer coefficient for a Nanofluid with 6.8% volume concentration. The increase of nanoparticle concentration in the base liquid resulted in the decrease of temperature in the heated block. Further nanofluids with 36nm particle size were showing more heat transfer compared to 47nm particle size. Chen et al. (2008) have studied synthesis, characterization and dispersion characteristics of Titanate nanotubes in water. In their experimental work they investigated the thermo physical properties such as the effective thermal conductivity, the viscosity and the forced convective heat transfer of the nanofluids. Li and Kleinstreuer (2008) investigated the thermal performance of 1-4% volume concentration CuO/water nanofluids flowing in a trapezoidal micro-channel. The results clearly indicated that the thermal performance of nanofluids increases with increase in the nanofluid volume fraction. However, nanofluids with higher particle concentration have shown considerable increase in the pressure drop

13 17 requiring more pumping power. They concluded that micro-channel heat sinks with nanofluids are considered to be the promising cooling devices. Koa et al. (2007) measured viscosity and pressure drop characteristics of nanofluids with carbon nanotubes present in the water and found that carbon nanotubes concentrations and preparation methods of nanofluids have a bearing on the pressure drop. Stable suspensions of nanotubes were prepared by two methods. In the first method a surfactant was used and in the second method acid was mixed in the nanofluids. Both the nanofluids have shown an increase in the nanofluid viscosity with decreasing shear rate and pressure drop. Both the nanofluids presented similar results when compared to base liquids because of shear thinning nature of CNT nanofluids. It was also observed from the study that CNT nanofluids produced low friction factor compared to the base fluids alone especially in the turbulent flow regimes. Namburu et al. (2008) carried out numerical study of turbulent flow and heat transfer characteristics of CuO, Al203 and SiO2 nanoparticles in base fluid of ethylene glycol and water blend applying constant heat flux boundary conditions. The results indicated that at a Reynolds number of the heat transfer coefficient of a 6% CuO nanofluids increased by 175% over the base fluid heat transfer coefficient. It is further noted that heat transfer coefficient is a strong function of nanofluid volume concentration and Reynolds number.

14 18 Employing of nanofluids at higher temperature gives higher percentage increase in the heat transfer enhancement. The results also showed that CuO nanofluids have exhibited higher heat transfer enhancement followed by Al203 and SiO2 nanofluids. 2.6 LITERATURE ON HEAT TRANSFER WITH INSERTS The twisted tape inserts are used in circular tubes to alter flow pattern of fluids. It is an established fact that insertion of twisted in circular tubes creates a swirl in the fluid flow and the flow behavior of the liquids is dynamically a different one. The swirl promotes transport of heat from tube wall to the surrounding fluid very effectively for all the flow regimes. Turbulence and swirl can be created in the fluid by different types of twisted tape inserts. Different researchers groups have studied the flow pattern and heat transfer characteristics of increased turbulence and developed correlations for Nusselt number. Smithberg and Landis (1964) studied the spiraling and fin effects of tapes on the characteristics of turbulence boundary layers. Thorsen and Landis (1968) have carried investigation on heat transfer and studied the effect buoyancy, temperature gradients and property variation on gases in heating and cooling process. Lopina and Bergles (1969) estimated mean heat transfer coefficient considering contributions of spiral and centrifugal convections factors and a correlation is developed taking into account the fin effect and a multiplication factor. The value of this multiplication factor depends

15 19 on whether the insert is held either loosely or tightly in the circular tube. Bergles and Web (1970) studied convective heat and mass transfer augmentation techniques. Bergles (1985) made a thorough review on heat and mass transfer augment techniques which are listed in the heat transfer hand book. Manglik and Bergles (1993) studied comprehensively on swirl flow in both the laminar and turbulent regimes by tape insertion in the tube and presented experimental data. The data was subjected to regression analysis and correlation equations related to the momentum and heat transfer and friction factor were developed for a swirl flow over a wide range of Reynolds numbers considering the tape thickness. Sarma et al. (1996) analyzed the convective heat transfer and pressure drop for high Prandtl number fluids with twisted tape turbulence promoters. Saha and Dutta (2001) investigated the effect of uniform and varying pitch twisted tapes on heat transfer and associated pressure drop in the laminar regimes of high viscous fluids flowing in a circular tube with constant heat flux as boundary conditions. The results shown that varying pitch twisted tapes give more pressure drop than the twisted tapes of uniform pitch. Kumar and Prasad (2000) investigated the effect of twisted tape inserts in solar water heater systems and observed that twisted tape inserts increases heat transfer coefficient. Klaczak (2001) in his experimental work, observed a considerable heat transfer enhancement with the insertion of twisted

16 20 tapes in vertical and horizontal pipes in laminar flow condition. Sarma et al. (2002) presented a new approach in predicting the forced convective heat transfer coefficient in a tube with twisted tape inserts of different pitch to diameter ratios. Sarma et al. (2003) postulated a new method to predict heat transfer coefficients with twisted tape inserts in a tube, in which the wall shear and the temperature gradients are properly modified through friction coefficient correlation leading to heat transfer augmentation from the tube wall. Sarma et al. (2004) proposed a method to estimate the average heat transfer coefficients in a tube of short length under laminar flow conditions. Sarma et al (2005) developed correlations to predict convective heat transfer coefficients and friction factors with twisted tapes inserts under wide range of Reynolds and Prandtl numbers and found that their correlations are in good agreement with correlations. Swith Eiamsa-ard and Pronget Promvonge (2006) observed that helical screw tape insert has significant effect on enhancing heat transfer rate and friction factor. 2.7 SCOPE OF THE PRESENT WORK It is observed from the literature review that thermo physical properties of nanofluids are evaluated by different research groups. Nanofluids with different metals and metal oxide nanoparticles were prepared using water as the base fluid in majority of earlier experimental studies. Heat transfer and friction factor studies on different nanofluids were carried out by researchers at and above

17 21 room temperature by varying nanofluid concentration. The variation of heat transfer coefficient with nanoparticle concentration and the Reynolds number for nanofluids flowing in a circular tube, in laminar and turbulent flow regimes were investigated as per the literature survey. The experimental work is also carried out to study on passive heat transfer enhancement in pure fluids using inserts. Ethylene glycol and propylene glycols are anti-freeze liquids and normally mixed in water in different proportions. Such blends are used as a heat transfer fluid in cold climatic conditions, to lower the aqueous freezing point of heat transfer fluids in automobile radiators and similar heat exchangers. Limited data is available on thermo physical properties and heat transfer characteristics of ethylene glycol based nanofluids at present. Heat transfer and friction factor studies on propylene glycol based nanofluids are not explored. Even presence of 30 % propylene in water freezes only at -13 C. Experimental work on passive heat transfer enhancement using inserts in propylene glycol based nanofluids for further augmentation of heat transfer is a grey area to be explored. Popylene glycol is an anti-freezing liquid; it is chemically more stable, non-toxic and gives a stabile suspension when compared to ethylene glycol. Propylene glycol and water mixture can be used as base or host fluid. Namburu et al. (2008) studied heat transfer characteristics metal oxides in ethylene glycol/water as the base liquid. No work is reported so far on the properties and heat transfer

18 22 characteristics of Propylene glycol based CuO nanofluids. The experimental heat transfer data for pure water with twisted tape and longitudinal strip inserts in laminar and turbulent flow conditions are also available in the literature. But, no experimental work is reported so far on heat transfer using propylene glycol as base liquid with inserts. This motivated the scholar to investigate the heat transfer and friction factor characteristics of propylene glycol based CuO nanofluids in different flow regimes. It is proposed to prepare CuO nanofluids and carryout experimental work and to measure temperature dependent thermo physical properties of CuO Nanofluids viz., thermal conductivity, density, absolute viscosity and specific heat. It is also proposed to study the effects of nanoparticle volume concentration and temperature on properties of CuO nanofluids. In the present investigation it is also intended to study the heat transfer and friction factor of CuO nanofluids flowing in a plain tube under constant heat flux as the boundary condition. For this it is proposed to prepare CuO nanofluids of three different volume concentrations using water-propylene glycol (70:30 by volume percentage) blend as the host fluid and to conduct experiments in laminar to transition conditions in the Reynolds number ranging from 1000<Re<10000 under constant heat flux as boundary conditions. It is proposed to compare the experimental heat transfer and friction

19 23 factor of nanofluids of three different concentrations with the correlations available in the literature. Heat transfer and friction factor characteristics of the same CuO nanofluids will be investigated by inserting twisted tape and helical inserts in the tube. The influence of nanoparticle concentration, mass flor rate and tape twist ratio on Nusselt number will be studied and discussed. Finally, generalized regression equations to predict Nusselt number and friction factor will be developed for the data that will be obtained in the experimental investigation. The experiments are proposed to be conducted for laminar and transition flow conditions in the same Reynolds number range for both the cases of plain tube and with inserts.