Experimental investigation of nanoparticles concentration, boiler temperature and flow rate on flow boiling of zinc bromide and acetone solution in a rectangular duct

Abstract

Despite the increase in heat transfer properties of nano-fluids, they are not currently used in vapour absorption refrigeration systems (VARS), and there is little literature on the flow boiling behaviour of concentrated salt solutions with nano-particle suspension. A potential novel working fluid solution for a vapour absorption refrigeration unit capable of utilising very low grade waste heat is acetone and zinc bromide, and this fluid is investigated here as the salt solution with graphene nanoparticles in suspension in flow boiling similar to that found in VARS. Nanoparticle concentration, boiler temperature, and flow rate are investigated. The Rohsenow constant in the flow boiling correlation for the nanofluid acetone/ZnBr2 with graphene on a stainless steel surface is found to be 0.217. By increasing the particle concentration from 0 to 05 vol%, heat flux and heat transfer coefficient on the heated surface increase from 8638 W/m2 and 106 W/m2 K to 13164 W/m2 and 167 W/m2 K, respectively. The steady pressure of the system increases with increasing loading of the nanoparticles and consequently the saturation temperature increases. This is because of the increased vapour generation as a consequence of improved heat transfer properties. Heat transfer coefficient is linearly proportional to temperature difference between the fluid and wall (e.g. increases from 78 W/m2 K to 145 W/m2 K when the temperature difference increase from 102 K to 135 K) in the range tested and the heat flux correspondingly reflects a quadratic relationship with temperature difference. Increasing nanofluid flow rate reduces both the production of acetone in the condenser and the salt concentration in the strong solution reservoir. Regarding properties of the fluid, the density and the specific heat follow the simple mixture combination rule; the thermal conductivity of the nanofluid increases by 4.5% with increasing the loading the particles to 0.5 vol%, following reasonably well the correlation of Suganthi et al. (2014); the viscosity increases linearly with concentration of nanoparticles (e.g. increases from 3.22 m Pa s to 4.5 m Pa s by increasing the concentration from 0 to 0.5 vol%); the stability of the nano-salt-fluid is affected by the density of the base fluid. The nanofluid showing good stability for 4 h and during the circulation of the fluid in the rig. Over the range of temperatures tested, the salt solution demonstrates characteristics of nucleate boiling behaviour and offers significant improvement over the properties of the base fluid in terms of boiling effectiveness, indicating that it will provide improved operation in a VARS situation.