Surface roughness is one of the most important factors to determine the flow and heat transfer characteristics of the microchannel. This paper experimentally and theoretically investigated the effects of surface roughness for the flow and heat transfer behavior within the circular microchannel. The stainless steel circular microchannels were fabricated by electrical spark-erosion perforating and drilling separately to control the relative roughness of the surface which is 1% for drilling method and 1.5% for electrical spark-erosion perforating method. Each test piece includes 44 identical circular microchannels in parallel with diameter of 0.4 mm. In the experiments, the air flowed through the circular microchannels with Reynolds number changing from 200 to 2600. The results showed that the surface roughness in microchannels has a remarkable effect on the performance of flow behavior and heat transfer within the circular microchannel. The values of Poiseuille number and Nusselt number are higher when the surface relative roughness is larger. At the same time, the flow behavior is inconsistent with the behavior within the macrochannel. For the flow behavior, Poiseuille number increases monotonously with the increase of Reynolds number, and is higher than the constant theoretical value. The Reynolds number for the transition from laminar to turbulent flow is between 1400 and 1600. For the heat transfer property, Nusselt number also increases as the increase of the Reynolds number.

In present study, an experimental investigation has been carried out to analyze the heat transfer characteristics of CuO-water nanofluids jets on a hot surface. A rectangular stainless steel foil (AISI-304, 0.15 mm thick) is used as a test surface is electrically heated to obtain the required initial temperature. The distribution of heat flux on the target surface is evaluated from the recorded thermal images during transient cooling. The effect of nanoparticle concentration and Reynolds number of the nanofluids jet impingement heat transfer characteristics is studied. Tests were performed for an initial surface temperature of 500°C, Reynolds number (5000≤Re≤13000), CuO-water nanofluids concentration (Φ= 0.15%, 0.6%) and nozzle to plate distance was l/d= 4.

The heat transfer performance of fluid flowing in a microchannel was experimentally studied, to meet the requirement of extremely high heat flux removal of microelectronic devices. There were 10 parallel microchannels with rectangular cross-section in the stainless steel plate, which was covered by a glass plate to observe the fluid flowing behavior, and another heating plate made of aluminum alloy was positioned behind the microchannel. Single phase heat transfer and fluid flow downstream the microchannel experiments were conducted with both deionized water and ethanol. Besides experiments, numerical models were also set up to make a comparison with experimental results. It is found that the pressure drop increases rapidly with enlarging Reynolds number (200), especially for ethanol. With comparison, the flow resistance of pure water is smaller than ethanol. Results also show that the friction factor decreases with Reynolds number smaller than the critical value, while increases the velocity, the friction factor would like to keep little changed. We also find that the water friction factors obtained by CFD simulations in parallel microchannels are much larger than experiment results. With heat flux added to the fluid, the heat transfer performance can be enhanced with larger Re number and the temperature rise could be weaken. Compared against ethanol, water performed much better for heat removal. However, with intensive heat flux, both water and ethanol couldn’t meet the requirement and the temperature at outlet would increase remarkably, extremely for ethanol. These findings would be helpful for thermal management design and optimization.

Surface with hierarchical structures to enhance heat transfer performance has been investigated for decades. Here we present a unique two-stage vaporization phenomenon observed when the heated surface is covered to confine the thickness of liquid film. This multi-stage vaporization will be critical for optimization applications in thermal management. The hierarchical surface is prepared using: a) isotropic etching to form micro-pillars, and b) wet-chemical etching to produce silicon nanowires (SiNWs) on sidewalls of micro-pillars. Oxygen plasma was applied to modify their surface wettability from superhydrophobic to superhydrophilic. A glass slide was placed right on top of the structures, covering about half of the heated surface, and mechanically clamped by a stainless-steel block. Therefore, the liquid film is about the same height as the micro-pillars. The hierarchical surface was wetted with deionized water before being heated. There are two clear and separate stages that were observed under an high-speed camera: i) thinning while liquid is vaporizing, and ii) mixture of vaporizing and condensation until total dry-out happens. The detailed dynamics of this two-stage vaporization were explored and reported in this study.

The clustering phenomenon on the solid wall during dropwise condensation is analyzed with reflection spectrum. From the theoretical prediction of the reflectivity for thin liquid films with different thickness on stainless steel surface, it is ascertained that the reflectivity corresponds to the coacervate characteristics of the steam molecular. Furthermore, the experimental data of the reflection spectrum during dropwise condensation in literature also demonstrated that the reflection feature and so as the coacervate characters lie between liquid and steam after the droplet departing during an actual continuous condensation process. The clustering model is used to analyze the results, indicating that clusters form on the blank surface. And it is found that different microstructures of the solid wall would lead to different deposition rates of the clusters, which prompts an effective way to enhance heat transfer process of condensation by accelerating the deposition rate of clusters with surface modification.

An experimental investigation of condensation heat transfer and pressure drop of ammonia flowing through a single, circular, microchannel ( D = 1.435 mm) was conducted. The use of ammonia in thermal systems is attractive due to its high latent heat, favorable transport properties, zero ozone depletion (ODP), and zero global warming potential (GWP). At the same time, microchannel condensers are also being adopted to increase heat transfer performance to reduce component size and improve energy efficiency. While there is a growing body of research on condensation of conventional refrigerants ( i.e. , R134a, R404A, etc.) in microchannels, there are few data on condensation of ammonia at the microscale. Ammonia has significantly different fluid properties than synthetic HFC and HCFC refrigerants. For example, at T sat = 60°C, ammonia has a surface tension 3.2 times and an enthalpy of vaporization 7.2 times greater than those of R134a. Thus, models validated with data for synthetic refrigerants may not predict condensation of ammonia with sufficient accuracy. The test section consisted of a stainless steel tube-in-tube heat exchanger with ammonia flowing through a microchannel inner tube and cooling water flowing through the annulus in counterflow. A high flow rate of water was maintained to provide an approximately isothermal heat sink and to ensure the condensation thermal resistance dominated the heat transfer process. Data were obtained at mass fluxes of 75 and 150 kg m −2 s −1 , multiple saturation temperatures, and in small quality increments (Δ x ∼15–25%) from 0 to 1. Trends in heat transfer coefficients and pressure drops are discussed and the results are used to assess the applicability of models developed for both macro and microscale geometries for predicting the condensation of ammonia.

Temperatures of the carbide tip’s surface when turning stainless steel with a chamfered main cutting edge nose radius tool are investigated. The mounting of the carbide tip in the tool holder is ground to a nose radius as measured by a toolmaker microscope, and a new cutting temperature model developed from the variations in shear and friction plane areas occurring in tool nose situations are presented in this paper. The frictional forces and heat generated in the basic cutting tools are calculated using the measured cutting forces and the theoretical cutting analysis. The heat partition factor between the tip and chip is solved by the inverse heat transfer analysis, which utilizes the temperature on the P-type carbide tip’s surface measured by infrared as the input. The tip’s carbide surface temperature is determined by finite element analysis (FEA) and compared with temperatures obtained from experimental measurements. Good agreement demonstrates the accuracy of the proposed model.