Mohammed, Hayder I.
(2019)
Numerical and experimental investigation of boiling nanofluids in vapour absorption refrigeration.
PhD thesis, University of Nottingham.
Abstract
To achieve decreasing power demands across several industries, improving the heat transfer of vapour absorption refrigeration system (VARS) based on flow boiling heat transfer have been proposed. Nanofluid, a fairly new class of coolants formed by suspending 1-100 nm sized particles in a base fluid which in this thesis was presented by a salt - fluid (acetone / ZnBr_2), has been shown to improve a fluid’s thermal properties. Previous researches concentrate on water based fluids or commonly found refrigerants and heat transfer fluids. This research focuses on developing a new method to simulate flow boiling of a novel fluid, we define it as a nano-salt-fluid, numerically. A model was generated using commercial CFD code with additional user defined function, (UDF) for the case of determining the heat and mass transfer in a boiler duct for a vapour absorption refrigerator. The simulations were performed to assess the effect of varying nanoparticle concentrations, fluid velocity and boiler temperature on the boiling and phase change characteristics of the system. The fluid was treated as four separate phases: liquid acetone; vapour acetone; liquid Acetone / ZnBr_2 solution; solid nanoparticles cloud. Graphene nanoparticles were represented as a cloud-like phase in the mixture to calculate the viscosity. This study evaluates the key characteristics of the nanofluid system, and how the different components and phases behave when the liquid acetone evaporates. The process was modelled using ANSYS Fluent V.15 using the mixture multiphase flow model, however, the volume of fluid (VoF) method is also used to show the behaviour of the vapour phase.
To validate the modelling method, an experimental work examines the improvement of the heat transfer and investigates the performance of the working fluid. The high pressure part of the VARS is built as the test rig. The rig evaluated the heat transfer improvement in the boiler side of VAR system by use of nanofluid instead of the normal binary fluid only, which is acetone/ZnBr_2 in this case. Various values of flow rate, temperature and the concentration of the nanoparticles were used and the effects observed on the heat transfer and the fluid flow. These tests had acceptable accuracy, for example, the heat flux and the heat transfer coefficient are 8704\pm 85 W/m^2 and 104\pm 0.5 W/m^2.K, respectively, when the boiler temperature is 180 oC. The tests validated the results of the numerical modelling part of this work, which simulated the boiling of the salt-nanofluids, in order to justify the use of the CFD on a design tool for VARS.
Nanofluid in CFD are represented as multiphase and single-phase. To model the boiling process, a comparison between these models was assessed to discover which model represents the nano-particles as a distinct fluid phase used in the boiling process. The cases of laminar and turbulent flow regimes were explored numerically for a wide range of acetone and nanoparticles concentrations. The velocity was varied between 1.5 – 6 m.s^{−1}, representing typical heat exchanger conditions. Reynolds number depended significantly on the solution concentration.
The novel working fluid solution consisted of dissolved salt in refrigerant mixed with nanoparticles, called have a nano-binary fluid (NBF) or nano-salt-fluid (NSF). The study covered the properties of the acetone/ ZnBr_2 as a binary (base) fluid and nanofluid by adding Zinc Oxide (ZnO) and Graphene, separately. The overall results show that the nanoparticle enhances the thermal properties of the fluid.
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