Abstract
The curved geometry of a coiled flow inverter (CFI) promotes chaotic mixing through a combination of coils and bends. Besides the heat exchanger geometry, the heat transfer can be enhanced by improving the thermophysical properties of the working fluid. In this work, aqueous solutions of dispersed TiO2 nanometer-sized particles (i.e., nanofluids) were prepared and characterized, and their effects on heat transfer were experimentally investigated in a CFI heat exchanger inserted in a forced convective thermal loop. The physical and transport properties of the nanofluids were measured within the temperature and volume concentration domains. The convective heat transfer coefficients were obtained at Reynolds numbers (NRe) and TiO2 nanoparticle volume concentrations ranging from 1400 to 9500 and 0–1.5 v/v%, respectively. The Nusselt number (NNu) in the CFI containing 1.0 v/v% nanofluid was 41–52% higher than in the CFI containing pure base fluid (i.e., water), while the 1.5 v/v% nanofluid increased the NNu by 4–8% compared to water. Two new correlations to predict the NNu of TiO2–water nanofluids in the CFI at Reynolds numbers of 1400 ≤ NRe ≤ 9500 and nanoparticle volume concentrations ranges of 0.2–1.0 v/v% and 0.2–1.5 v/v% are proposed.
Highlights
IntroductionHeat exchangers play a vital role in a wide range of industries, such as the pharmaceutical, food and beverage, chemical, petrochemical, oil and gas, power generation, HVAC-R (heating, ventilation, air conditioning, and refrigeration), and other fields
Heat exchangers play a vital role in a wide range of industries, such as the pharmaceutical, food and beverage, chemical, petrochemical, oil and gas, power generation, HVAC-R, and other fields
Kumar and Nigam [25] characterized the hydrodynamics and forced convection in a coiled flow inverter (CFI) set, and compared them to those that occur in a straight coil configuration. Both scenarios were studied under laminar regime conditions, and the results revealed that the bent coil configuration (i.e., CFI set) showed a 20–30% enhancement in Nusselt number (NNu) when compared to that of the straight coil
Summary
Heat exchangers play a vital role in a wide range of industries, such as the pharmaceutical, food and beverage, chemical, petrochemical, oil and gas, power generation, HVAC-R (heating, ventilation, air conditioning, and refrigeration), and other fields. Industry-specific requirements have led to the development of enhanced heat exchangers capable of transporting high heat fluxes without compromising on practical sizing aspects. With regard to passive and active heat transfer enhancement techniques [1], the former are commonly used due to their lower cost, their relatively easy implementation, and their longer operating life [2]. The improvement of the thermophysical properties of working fluids, as a passive enhancement technique, offsets the low thermal property of conventional heat transfer fluids (e.g., water, ethylene glycol, and engine oil). In 1995, Choi and Eastman [3] proposed a novel class of engineered heat transfer fluids called nanofluids (NFs), in which nanometer-sized particles of materials with high thermal conductivity are dispersed in a base fluid. Many researchers have experimentally determined the thermal conductivity (k) of different NFs with typical volume concentrations in the 0.5–4.0 v/v%
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