Richard Furberg

The Use of a Nano- and Microporous Surface Layer to Enhance Boiling in a Plate Heat Exchanger

Presented research is an experimental study of the performance of a standard plate heat exchanger evaporator, both with and without a novel nano- and microporous copper structure, used to enhance t ...

Experimental Investigation of an Evaporator Enhanced With a Micro-Porous Structure in a Two-Phase Thermosyphon Loop

Following is an experimental study of six different evaporators in a closed two-phase thermosyphon loop system, where the influence of various evaporator dimensions and surfaces was investigated. The evaporators featured a 30 mm long rectangular channel with hydraulic diameters ranging from 1.2–2.7 mm. The heat transfer surface of one of the tested evaporators was enhanced with copper nano-particles, dendritically connected into an ordered micro-porous three dimensional network structure. To facilitate high speed video visualization of the two-phase flow in the evaporator channel, a transparent polycarbonate window was attached to the front of the evaporators. Refrigerant 134A was used as a working fluid and the tests were conducted at 6.5 bar. The tests showed that increasing channel diameters generally performed better. The three largest evaporator channels exhibited comparable performance, with a maximum heat transfer coefficient of about 2.2 W/(cm2 K) at a heat flux of 30–35 W/cm2 and a critical heat flux of around 50 W/cm2 . Isolated bubbles characterized the flow regime at peak performance for the large diameter channels, while confined bubbles and chaotic churn flow typified the evaporators with small diameters. In line with previous pool boiling experiments, the nucleate boiling mechanism was significantly enhanced, up to 4 times, by the nano- and micro-porous enhancement structure.Copyright

Advanced Thermosyphon Cooling With Nanoporous Structured Mini Channel Evaporators

Attention has been given to enhance boiling surfaces in order to decrease the temperature difference and to increase heat transfer coefficient. Structured surfaces may provide both surface enlargement and artificial nucleation sites, thus ameliorate the heat transfer coefficient. The goal of the present experimental work is to analyze the influence on heat transfer coefficient (HTC) of enhanced surface structures coated on mini channel heat exchanger working in a closed loop thermosyphon system. Experimental tests were carried out with three types of surface enhanced mini channel evaporators: smooth surface, threaded structure and nanoporous coating. The evaporators are single channel half circularly shaped, adapted for filming purpose, measuring 30mm in length and 3mm in diameter. Surface areas of channels are 1.41cm2 . Experiments were conducted in refrigerant 134a at 4.87bar (reduced pressure pr = 0.12) and at heat fluxes ranging from 0.7W/cm2 to 63.8W/cm2 . A high speed video camera was used for visualization of the two-phase flow in the evaporator channel. It is shown that threaded surface provides the highest heat transfer coefficient (HTC) from no load to heat flux of 7.1W/cm2 , the nanoporous structure shows the highest performance between 7.1W/cm2 and 49.6W/cm2 , and the smooth surface channel exhibits the best HTC from 49.6W/cm2 and higher. In this paper, the influences of heat flux and surface structures on HTC are discussed, and the impact of refrigerant flow regimes on heat transfer performance is also highlighted.Copyright