Pool boiling heat transfer characteristics of structured surfaces

Abstract

The present dissertation reports experimental investigations pertaining to the pool boiling performance of test surfaces developed by using wire-EDM, nanosecond pulsed laser, continuous wave laser with different pool liquids such as water, isopropyl alcohol and acetone. The objective of the present work is to analyze the effects of variation of laser type, power and wavelength on physical characteristics of the test surface. Also, the pool boing performance of surfaces, developed by wire-EDM method and laser processing technique, are studied for a varied range of input heat fluxes and different pool liquids. Design and development of a test facility involving various modules such as cylindrical boiling container, heater assembly, condensing coil, power supply system and temperature measurement scheme have been made to study the pool boiling heat transfer characteristics of smooth and different structured surfaces. Initial tests are conducted at atmospheric pressure with saturated condition for a smooth copper surface. The input heat supplied to the test section is calculated by measuring the voltage and current values across the heater. The surface temperature of the boiling surface is calculated by using the measured temperature data of the test section and input heat flux. Subsequently, the heat transfer coefficient is evaluated by using the input heat flux and wall superheat values. Four different structured surfaces with different structured geometries are fabricated by employing wire-EDM machining process and their boiling performance is studied for heat flux input in the range of 0-300 kW/m2 for water and 0-250kW/m2 for isopropyl alcohol. The pool boiling performances of structured surfaces are compared with the smooth surface. Also, the mechanisms of boiling phenomena in structured surfaces are discussed. Next, multiscale functionalized copper surfaces are fabricated by employing nano-second pulsed wave laser for a varied range of laser power and laser wavelength settings (1064 nm, 532 nm and 355 nm). The pulsed laser is a Nd:YAG (Quanta-Ray, Spectra Physics) that produces 5-8 nanosecond pulses with a wavelength of 1064 nm at a repetition rate of 10 Hz. The physical characteristics of the fabricated test surfaces are studied by using stereomicroscopic images (EZ4HD), scanning electron microscope (SEM) (Supra55, Zeiss) images and 3D profile of the test section. The pool boiling characteristics of nanosecond pulse laser processed surfaces are analyzed for different pool liquids, varied range of wall heat flux values and different laser powers (4.6 W, 4.2 W, 4.0 W, 1.8 W, 1.6 W, 1 W and 0.8 W). In addition to this, six copper test sections are developed by using continuous wave laser for a varied range of laser power (40-50W). The continuous wave laser used in the present investigation is Ytterbium (Yb)-doped fiber laser (IPG Photonics) and is installed in a laser marking machine (SCANTECH, India). The physical characteristics of the test specimens developed by using continuous wave laser are studied by using SEM images. Also, the pool boiling characteristics of laser processed test surfaces are compared with the smooth surface. The results obtained by the present experimental study are compared with the test results obtained by previous researchers. Keywords: heat flux, pool boiling, enhanced surfaces, laser ablation, nucleate boiling, heat transfer coefficient, microstructure, structured geometries