NUCLEATE POOL BOILING OF NANOPARTICLE BASED FLUIDS

Abstract

This thesis pertains an experimental investigation on nucleate pool boiling of distilled water and nanofluids namely, alumina – distilled water and copper oxide – distilled water on stainless steel heating tube surface at atmospheric and sub atmospheric pressures. Basically, it deals with the synthesizing of alumina – distilled water and copper oxide – distilled water nanofluids and their characterization. It also deals with the effect of operating variables namely, heat flux, pressure and concentration of nanoparticles on the heat transfer coefficient for the pool boiling of nanofluids. Further, it also includes the comparison of heat transfer coefficient of nanofluids with that of distilled water. Finally, an empirical dimensionless correlation for calculation of heat transfer coefficient of nanofluids has been developed. Experiments have been carried out for synthesizing of the nanofluids as per the standard procedure as mentioned in section 3.2. The thermophysical characteristics namely density, viscosity and thermal conductivity of alumina-distilled water and copper oxide-distilled water nanofluids have been studied. The experimental data of density of these two nanofluids have been generated for various concentrations of nanoparticles at atmospheric pressure and an empirical correlation is developed as given below: ρeff = C1+C2φp+C3 The value of these constant depends upon the type of nanofluids. The predicted values of density matches excellently well within an error of + 2.5%. T The experimental data of thermal conductivity of both the nanofluids have been generated for various concentration of nanoparticles at various temperatures. The value of thermal conductivity has been found to vary with temperature and concentration of nanoparticles according to power law relationship as given below: 𝑘𝑒𝑓𝑓𝑘𝑏𝑓∝ 𝜑𝑎𝑇𝑏 The prediction of thermal conductivity matches with the experimental values within an error of + 6%. ii The experimental data on viscosity have been generated for various concentration of nanoparticles in nanofluids for different temperature and a logarithmic relationship has been developed by regression analysis which is given below:ln (𝜇𝑛𝑓)=𝐴𝑇−𝐵 The predicted values for viscosity using correlation matches excellently within an error of + 7%. Experiments have been carried out for the pool boiling of distilled water, alumina – distilled water and copper oxide – distilled water nanofluids on an electrically heated stainless steel heating tube surface. The heating tube is a stainless steel cylinder having 18 mm inner diameter, 32 mm outer diameter and 150 mm effective length. It is heated by placing a laboratory made electric heater inside it. The wall temperature and liquid pool temperature are measured by using well calibrated polytetrafluoroethylene (PTFE) coated copper – constantan thermocouples. The thermocouples are placed inside four coaxially drilled holes at a pitch circle diameter of 25 mm for the measurement of surface temperature. Similarly thermocouple probes are placed in liquid pool corresponding to wall thermocouple positions in heating tube to measure the liquid pool temperature. A digital multimeter measures emf of thermocouples. Power input to the heater is increased gradually from 240 W to 440 W in six equal steps and pressure from 45.47 kN/m2 to 97.71 kN/m2 Experimental data for saturated pool boiling of distilled water on stainless steel heating tube at atmospheric and sub atmospheric pressures have been processed to obtain local as well as average heat transfer coefficient. Analysis of the data has shown that surface temperature, for a given value of heat flux, increases from bottom to side, to top positions of heating tube at atmospheric and sub atmospheric pressures. However, liquid temperature remains almost constant. Further, the local heat transfer coefficient increases from top to side to bottom positions irrespective of heat flux and the heat transfer coefficient has been found to vary with heat flux according to power law relationship, ℎ𝜓∝𝑞0.7 for all the values of pressures. Furthermore, average heat transfer coefficient of distilled water boiling on stainless steel heating tube has been found to vary according to the relationship ℎ∝𝑞0.7 for atmospheric and sub atmospheric pressures. This corroborates with the findings of various researchers such as [ A10, A11, B3, C10, L7, M1, T4, Y4 ].A dimensional equation for heat transfer coefficient has been developed as ℎ=𝐶1𝑞0.7𝑝0.32 using in five steps. The maximum uncertainity associated with the measured value of average heat transfer coefficient is of the order of + 1.69%. iii regression analysis for the pool boiling of distilled water on stainless steel heating tube surface, where C1 Experimental data for the boiling of alumina distilled water and copper oxide – distilled water nanofluids at atmospheric and sub atmospheric pressures on stainless steel heating tube surface resulted in analogous behavior as that of distilled water. The functional relationship of heat transfer coefficient with heat flux and pressure is same as observed for distilled water and therefore a dimensional equation, ℎ= 𝐶2𝑞0.7𝑝0.32 for the boiling of nanofluids at atmospheric and sub atmospheric pressures has been developed by regression analysis within an error of + 9% ; where Cis a constant whose value depends on the type of liquid and the surface characteristics of heating tube. 2 Comparison of boiling characteristics of distilled water and the two nanofluids has been carried out. The pool boiling heat transfer coefficient enhances with increase in concentration of both alumina and copper oxide nanoparticles in distilled water. This behavior continues upto a certain optimum value of concentration of nanoparticles in distilled water. The maximum enhancement of 52.76% and 30.71% is obtained in heat transfer coefficient in case of alumina – distilled water and Copper Oxide – Distilled Water respectively, at 0.05% by concentration of both the nanoparticles in the distilled water. However, on further increasing the concentration of nanoparticles in base fluid i.e. distilled water yields deterioration in the boiling heat transfer coefficient. These facts were also corroborated by various researchers such as Das et. al. [D1,D2] , Kwark et.al. [K17 ] for the boiling of nanofluids beyond critical concentration and White et. al. [W10 ] in their work. is a constant whose value depends upon the concentration of nanoparticles in the base fluid and heating surface characteristics of heating tube. These observations also corrobates with the findings of various researchers such as Park & Jung [P2]; Kole and Dey [K14]; Kim et. al. [K11]; Johnathan and Kim [J4]; Sarfaraz and Peyghambarzadeh[S2] and Wen and Ding [W5,W6 ] over boiling of different compositions of nanofluids. A dimensionless correlation has been developed to estimate the heat transfer coefficient for pool boiling of nanofluids Nu = 3.709x10-4 Pr1.32QP0.017Ja-0.97. This equation has been compared with the data of present investigation as well as other investigators namely Ceislinski [C7, C8]; Kole and Dey [K14 ]; Bang & Chang [ B2]; Wen & Ding [W5,W6 ]; Ding & Chen [C2] and Yang & Liu [Y2]. The comparison between the experimental values and predicted values due to correlation match excellently well within an error of + 10%.