Investigation of the critical heat flux in small-diameter channels

The paper describes experimental studies into the hydrodynamics and heat exchange in a forced water flow in small-diameter channels at low pressures. The timeliness of the studies has been defined by the growing interest in small-size heat exchangers. Small-diameter channels are actively used in components of compact heat exchangers for present-day engineering development applications. The major difficulty involved in investigation of heat-transfer processes in small-diameter channels consists in the absence of common methodologies to calculate coefficients of hydraulic resistance and heat transfer in a two-phase flow. The channel size influences the heat exchange and hydrodynamics of a two-phase flow as one of the determining parameters since the existing internal scales (vapor bubble size, liquid droplet diameter, film thickness) may become commensurable with the channel diameter, this leading potentially to different flow conditions. It is evident that one cannot justifiably expect a change in the momentum and energy transfer regularities in single-phase flows as the channel size is reduced for as long as the continuum approximation remains valid. The authors have analyzed the experiments undertaken by Russian scientists to investigate the distribution of thermal-hydraulic parameters in channels with a small cross-section in the entire variation range of the flow parameters in the channel up to the critical heat flux conditions when the wall temperature increases sharply as the thermal load grows slowly. The experimental critical heat flux data obtained by Russian and foreign authors has been compared.

Dimensional Characterizations Using Scanning Electron Microscope and Surface Improvement With Electrochemical Polishing of Additively Manufactured Microchannels

Microchannel manufacturing is one of the fastest growing areas in advanced manufacturing with numerous applications, including turbine blade cooling structures, compact microchannel heat exchangers, and electronic cooling devices. Recent development of metallic additive manufacturing (AM) based on direct metal laser sintering technology is capable of fabricating microscale structures with high complexity and design flexibility. However, powder bed laser sintering process produces rough surface characteristics caused by hatch overlaps and particle attachments, leading to channel size reductions and rough surfaces. In this paper, dimensional metrology of cross-sectional views of multirow microchannels made by AM was conducted by a scanning electron microscope (SEM) at different locations along the printing direction. Channel size reduction, surface roughness, and circularity tolerance of the as-printed channels were analyzed based on micrographs captured by SEM. Results showed that both channel sizes and hole pitches affected the printing qualities of microchannels. The as-printed channel sizes reduced by more than 15% compared to the designed values. Two approaches were made in this paper to improve printing qualities. The first one was to redesign channel size in computer-aided design (CAD) model to make the as-printed channel sizes closer to the objective values. Electrochemical polishing (ECP) was then applied as a second way using sulfuric acid solutions. Surface roughness value was reduced by more than 40% after the ECP process.

Maximization of the capillary pump efficiency in microfluidics

This paper studies the efficiency of capillary pump analytically in circular, square and rectangular channels with results verified by experiment. The effect of liquid momentum is analyzed with respect to channel size and equations are developed to enable most efficient fluid pumping. It is found that the momentum term is negligible at channel cross-cut area < 0.1 mm2 while it has a significant contribution at > 0.3 mm2 region. The optimized equations show that the most efficient pumping and thereby the quickest liquid filling is accomplished in square shaped channel when compared with rectangular and circular channels. Generally, the longer the filling distance, or the longer the filling time, the larger the channel size is required after optimization, and vice versa. For the rectangular channel with channel height fixed, the channel width requirement to maximize the ability of capillary pump is obtained and discussed. Experimental verifications are conducted based on the measurement of filling distance versus time, and the simulation results are well correlated with the testing results. The equations developed in the paper provide a reference for the microfluidic channel design, such that the channel filling speed can be maximized.