Heat Transfer Performance Of Surfaces Research Articles

Experimental investigation to quantify the influence of single artificial indentations on vapour bubble dynamics and the associated heat transfer rates

Dependence of single vapour bubble dynamics and the associated heat transfer phenomena on the type of artificial indentations created on the heated substrate has been studied. Experiments under nucleate boiling regime have been carried out for conical indentations of varying cone angles (2γ = 30, 60 and 90o) and cylindrical cavity keeping the depth of features constant (hc = 500 μm). Rainbow schlieren deflectometry has been employed for simultaneous mapping of bubble dynamics and thermal field in a completely non-intrusive and real time manner. Direct comparison of the rainbow schlieren images shows a relatively large partition of thermal energy through natural convection for conical indentation of small cone angle and cylindrical cavity. For large-angled conical indentation, increased availability of thermal energy for bubble growth process results in the formation of relatively larger-sized vapour bubble at a relatively slower rate. In addition, due to the relative differences in the amount of vapour entrapped inside the cavities between any two bubbling cycles, the consistency of bubbling features was found to be much higher for larger cone angled cavity. The evidence for increased partition of thermal energy transfer through natural convection has been seen in the form of an increase in the portion of superheat layer in the vicinity of vapour bubble for conical cavity with small cone angles from the whole-field temperature reconstructions. For ΔTsup = 10 K, the heat transfer performance of surfaces with conical cavity with large cone angle (2γ = 90o) is significantly much better than the other conical and cylindrical cavities.

Pool boiling heat transfer on superhydrophobic, superhydrophilic, and superbiphilic surfaces at atmospheric and sub-atmospheric pressures

Surface wettability is one of the key parameters in the manipulation of the boiling phenomenon. Although there are a number of studies on the effect of surface wettability on boiling heat transfer, there are few research efforts to explain the boiling phenomenon on superbiphilic surfaces at sub-atmospheric pressures. In this study, pool boiling experiments were conducted to investigate the boiling heat transfer performance of surfaces with uniform (superhydrophobic and superhydrophilic) and mixed (superbiphilic) wettability. This study presents the results obtained from four different surfaces for both atmospheric (103.7 kPa) and sub-atmospheric (28.3 kPa) pressures and aims to provide an understanding of the wettability effect using saturated deionized water as the working fluid in the heat flux range of 7 - 290 kW/m2. The experimental results show that the superbiphilic surface (superhydrophobic spots with a pitch size of 3 mm and diameter of 0.7 mm) offers improvements in boiling heat transfer at both atmospheric and sub-atmospheric pressures up to 98% and 54%, respectively. Due to bubble coalescence being more likely to occur at sub-atmospheric pressure, the enhancement effect of superbiphilicity on boiling heat transfer is more significant for atmospheric pressure.

Effect of T-shaped micro-fins on pool boiling heat transfer performance of surfaces

The saturated pool boiling heat transfer performance on the surface of T-shaped micro-fins was investigated with deionized water as working medium. Ten different T-shaped micro-fins surfaces were fabricated by machining method and the pool boiling experiments were carried out. By comparing the influence of varios parameters on the boiling heat transfer performance, the results showed that reducing the width of upper fin, increasing the gap or height of upper fin can improve the boiling heat transfer coefficient. With the increase of the width of the lower fins, the boiling heat transfer performance showed a different trend, that is, it increased at first and then decreased. The maximum boiling heat transfer coefficient of the surface S1 to the smooth surface was 184.7%. The visualization results showed that the merging of bubbles on the outside of the channel surface could strengthen the vertical and horizontal replenishment of the liquid to dry spot in the channel, thereby delaying the occurrence of critical heat flux and effectively reducing the wall temperature. The simplified semi-analytical model to determine the total heat flux of the micro channeal was proposed, which provided a heat flux prediction with an error of ± 40%.

Cicada-inspired self-cleaning superhydrophobic surfaces

Abstract Water-based heat transfer surfaces in HVAC&R (Heating, Ventilation, Air Conditioning and Refrigeration) systems have biofouling or microbial growth which might not only impose adverse effects to public health but also deteriorate the heat transfer performance of surfaces. In nature, a living creature is surviving while being exposed to formidable condition such as bacteria and fungi. Cicada wing has known to have multi-functional wings exhibiting nonwetting, and anti-microbial due to nanoscale pillars and biochemicals to achieve non-wetting characteristics. We conducted experiments on cicada wings and cicada-inspired engineered superhydrophobic samples to show that droplets jumping from superhydrophobic wings could carry away individual particles (~100 µm) or cluster of particles. This study was investigated on self-cleaning mechanism using jumping droplet condensation and elucidate further antifouling characteristics of non-wetting heat transfer surfaces.

Effects of Hydrophobic-Spot Periphery and Subcooling on Nucleate Pool Boiling from a Mixed-Wettability Surface

The effect of subcooling and length of hydrophobic-spot periphery on nucleate pool boiling heat transfer from TiO2-coated surface with and without PTFE (polyetatafluoroethylene) hydrophobic circle spots at intermediate heat flux has been examined. The experiments are performed with liquid subcooling ranging from 0-20 K and heat transfer block used were TiO2-coated copper block with a PTFE hydrophobic circle spot with various diameters with total area of PTFE being constant. Bubble nucleation and behavior were observed by using high-speed camera. The results showed that the heat transfer performance of surfaces with PTFE hydrophobic circle spot is better than superhydrophilic surface in overall condition. Furthermore, the heat transfer performance decreases under subcooled condition for all surfaces. Increase in peripheral length of hydrophobic-spot enhances the heat transfer performance.