TURBULENT FLOW AND HEAT TRANSFER AUGMENTATION IN MICROCHANNEL HEAT EXCHANGERS USING TWISTED-TAPE INSERTS: A NUMERICAL AND EXPERIMENTAL INVESTIGATION

Abstract

Microchannel heat exchangers (MCHEs) are central to thermal management of high-power-density electronics, concentrated solar receivers, and compact chemical reactors. This dissertation investigates the use of twisted-tape inserts (TTIs) as a passive heat transfer enhancement technique in rectangular microchannels (hydraulic diameter Dʰ = 0.8 mm) under turbulent flow conditions (Re = 5000–30000) with water as the working fluid. Three-dimensional Reynolds-Averaged Navier–Stokes (RANS) simulations using the k–ω SST turbulence model were performed in ANSYS Fluent 2024 R1 for twist ratios y = 2, 3, and 5 (pitch-to-diameter). Results reveal that the y = 2 tape yields Nusselt number augmentation of 2.8× at Re = 20000 relative to the plain channel, accompanied by a friction factor penalty of 4.1×, yielding a thermal performance factor (TPF) of 1.74. Experimental validation using a dedicated microchannel test rig with infrared thermography and precision differential pressure transducers confirmed CFD predictions within ±9% for Nu and ±11% for f across all conditions. Entropy generation analysis identified the optimal twist ratio as y = 3 (TPF = 1.81) from a second-law efficiency standpoint. Correlations for Nu and f as functions of Re and y are developed and proposed for design use.