C. S. Yang

Fabrication and design of a heat transfer micro-channel system by a low temperature MEMS technique

This work describes a low temperature fabrication technique for a micro-channel configuration that is integrated with an array of temperature sensors and micro-heaters. This channel configuration can be used to study the micro-scale heat transfer process in the channels. The low temperature fabrication process enables the use of low thermal conductivity material for forming the channel walls. This can provide a uniform heat flux boundary condition due to adequate insulation for reducing both the heat loss from the channel to the surrounding ambient and the streamwise conduction. In addition, the wall roughness of the micro-channel is minimized by a special wet etch process. Therefore, more accurate local heat transfer data along the channel can be obtained, which provides entry length heat transfer information for the first time in the literature. Design consideration and fabrication techniques used in this study are explained. Final measurements for validation of the fabricated heaters and sensors are presented. The paper also presents averaged heat transfer which is compared with available data. Discrepancies between different works can be clarified.

Numerical and Analytical Study of Reversed Flow and Heat Transfer in a Heated Vertical Duct

Both analytical and numerical calculations are performed to study the buoyancy effect on the reversed flow structure and heat transfer processes in a finite vertical duct with a height to spacing ratio of 12. One of the walls is heated uniformly and the opposite wall is adiabatic. Uniform air flow is assumed to enter the duct. In the ranges of the buoyancy parameter of interest here for both assisted and opposed convection, a reversed flow, which can be observed to initiate in the downstream close to the exit, propagates upstream as Gr/Re 2 increases. The increase in the Reynolds number has the effect of pushing the reversed flow downstream. Simple analytical models are developed to predict the penetration depth of the reversed flow for both assisted and opposed convection. The models can accurately predict the penetration depth when the transport process inside the channel is dominated by natural convection. Local and average Nusselt numbers at different buoyancy parameters are presented. Comparison with the experimental data published previously was also made and discussed. Good agreement confirms many of the numerical predictions despite simplifications of the numerical process made, such as two-dimensional and laminar flow assumptions.

Flow and heat transfer of natural convection in horizontal annulus with a heating element on inner cylinder

Experiments have been performed to study natural convection flow and heat transfer in a horizontal annulus when a square heating element is positioned at different locations on the inner insulated cylinder. The annulus is filled with water and has cylinder to cylinder diameter ratio of 3. The square heating element is small and has the width to annulus gap width ratio of 1/6. The range of Rayleigh number studied is approximately from 1.9 × 10 6 to 3.3 × 10 7 . It has been found that the flow pattern, the temperature distribution around the inner cylinder wall, and the local heat transfer rate around the outer cylinder are very sensitive to the location of the heating element. The heating element Nusselt numbers at various locations on the inner cylinder are obtained and well correlated against the Rayleigh number to the 1/3 powers. A maximum in the correlation parameter C is obtained when the heating element is placed 90 deg from the bottom.

Film-Cooling Performance and Heat Transfer over an Inclined Film-Cooled Surface

Experiments have been performed to study and obtain the adiabatic wall film-cooling effectiveness and the heat transfer over a film-cooled surface that is made inclined at various angles with respect to a uniform flow. The vertical temperature distribution was measured to infer the flow structure and the rate of mixing of the film jet with the freestream. The inclination angle of the film-cooled surface ranges from -20 to 20 deg with an increment of 5 deg, the blowing parameter from 0.5 to 2.0 (0.5, 0.7, 1.0, 1.5,2.0). It is found that the mixing of the film jet with the freestream is significantly affected by the inclination angle of the film-cooled surface. The divergence of the film-cooled surface can make the flow unstable and cause flow separation, whereas the convergence has both stabilization and impingement effects on the film jet. This has a very complicated effect on both the film-cooling performance and the heat transfer under the film. More detailed discussion on the flow and the heat transfer is presented. Correlations for both the film-cooling effectiveness and the heat transfer under the film are provided.

Fluid Flow and Heat Transfer in a Horizontal Channel With Divergent Top Wall and Heated From Below

Secondary flow structure and its enhancement on the heat transfer in a horizontal divergent channel have been studied. The bottom wall is horizontal and is heated uniformly while the opposite top wall is insulated and inclined with respect to the horizontal plane so as to create a divergent angle of 3 deg. At the entrance of the channel, the aspect ratios for the width to the height and the channel length to the height are 6.67 and 15, respectively. The Reynolds number ranges from 100 to 2000 and the buoyancy parameter Gr/Re 2 from 0 to 405. Both flow visualization and temperature fluctuation measurements at different locations are made to indicate the flow structure and oscillation of the secondary flow. The adverse pressure gradient in the divergent channel causes a thicker heated layer in the bottom and earlier initiation of secondary flow. Interaction between neighboring vortices and plumes becomes more severe and highly unstable. This precludes the formation of steady two-dimensional longitudinal vortex rolls in the downstream and leads to an earlier and larger enhancement of the heat transfer than the case of the parallel-plate channel. The effects of the buoyancy parameter and the divergence of the channel on the secondary flow structure and the Nusselt number are presented and discussed. DOI: 10.1115/1.4001606

Enhancement of heat transfer over a cylinder by acoustic excitation

Experiments were performed to demonstrate enhancement of heat transfer around a horizontal cylinder by the presence of acoustic excitation. The horizontal cylinder was heated uniformly and placed inside a wind tunnel. The wall temperatures around the cylinder were measured and are used to determine the local heat transfer in the circumferential direction. To avoid interference with the flow, the acoustic generator, which was a loud speaker, was placed downstream of the cylinder. The frequency ofthe sound F,, was set equal to the natural frequency, F n , of the shedding vortex in the wake or a multiple of F n . Therefore, synchronization of vortex shedding with acoustic wave can be expected. The excitation frequencies selected were F e /F n = 1, 2, 3, up to 8. Other frequencies at F e /F n = 1.5, 2.5, 3.5, up to 7.5 were also selected for comparison. During the experiments, the sound pressure varied from 0 to 100 dB and the Reynolds number varied from 2938 to 8814. The heat transfer around the cylinder was found to be significantly enhanced by the acoustic waves. More detailed measurements for the energy spectrum of the acoustic waves generated by the current speaker were made. This provides a better understanding of the physical process. Flow visualization is also performed to demonstrate synchronization of vortex shedding with acoustic excitation. The effect of sound pressure levels and Reynolds numbers on the wall heat transfer are presented and discussed.

Development of hybrid drive with observer controlled injection shot for the die casting machine

This paper propose a hybrid drives for die-cast machine in which combining hydraulic closed-loop injection shot and the other fully servo motor contrilled process. Based on hydraulic proportional valve position control, injection shot could be travelled with auto-tuning by means of pressure state observor in cave of die mold. In order to apply in die casting controller, it is required to design a Human Machine Interface (HMI) for parameter setting including basic injection travel, clamping force and die stroke from motion control of servo motor driving system.

Hybrid organic solar cells based on nanoparticles of copper indium sulfur and conjugated polymer

Photovoltaic devices fabricated from blends of copper indium sulfur (CIS) and poly (3-hexylthiophene) (P3HT) are demonstrated and cell performance is measured. The CIS material was made into nanocrystalline particles by polyol route. Both fabrication processes and parameters are adjusted to achieve optimal cell performance. These include selection of four different solvents used for dissolving both the P3HT and CIS for the formation of active layer, different concentration of CIS mixed into the polymer material leading to proper dispersion of the nanoparticles, and different baking temperatures used to vaporize the solvent. Optimal processing conditions are discussed. However, more effort is needed to increase the energy conversion efficiency.

Characterization of a n-type Field Effect Transistor made from direct growth and patterning of single wall carbon nanotubes film

Characterization of a n-type Field Effect Transistor (FET) made from direct growth and patterning of a dense single wall carbon nanotubes (SWNTs) network on a silicon substrate, using alcohol as source gas, is presented. This SWNTs network film made into FET has a special feature which is significantly different from the amorphous silicon thin film transistor (TFT) or the MOSFET. The primary n-type nature of the SWNTs FET exhibits ambipolar characteristic. In addition, different sizes of channel for the FET have been made to examine if the scaling law used in TFT or MOSFET applicable. The results found that unlike the mobility of silicon film measured in TFT or MOSFET which do not depend on the size of the channel, both the mobility and the transconductance of the SWNTs film measured in FET increases with the channel width. The current device has an advantage to improve the mobility simply be making a wider channel. More discussions on the characteristics of the SWNTs film FET are provided.

Fabrication of a Complicated Heat Transfer Microchannel System for CPU Cooling Study

Fabrication of a temperature sensor array and a heater in a complicated heat transfer microchannel system, on a glass substrate, is presented. In addition to the glass substrate, low thermal conductivity materials such as SU-8, PMMA plate are selected to form the micro channel wall to provide good thermal insulation. Both the titanium and the platinum are deposited sequentially on a glass plate and made into heater and sensors, respectively. Lift-off method is used to obtain the desired pattern for heater and sensors. The fabrication process is much simpler than those reported in literature. Design consideration and fabrication techniques involved in this processes is discussed. Finally, validation of the heaters and sensors fabricated and a measurement of the heat transfer coefficient distributions inside the micro channel is presented.