Minking K. Chyu

A benchmark study on the thermal conductivity of nanofluids

This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band (±10% or less) about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are (small) systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. [J. Appl. Phys. 81, 6692 (1997)], was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

Film Cooling Effectiveness and Heat Transfer Coefficient Distributions Around Diffusion Shaped Holes

We present an experimental study focusing on the effects of diffusion hole-geometry on overall film cooling performance. The study consists of three different but closely related hole shapes: (1) Shape A: straight circular hole with a 30 deg inclined injection, (2) Shape B: same as Shape A but with a 10 deg forward diffusion, and (3) Shape C: same as Shape B with an additional 10 deg lateral diffusion. The blowing ratios tested are 0.5 and 1.0. The density ratio is nominally equal to one. Measurements of the experiment use a transient liquid crystal technique that reveals local distributions of both film effectiveness (η) and heat transfer coefficient (h). The data obtained indicate that Shape C with a combined forward and lateral diffusion produces a significant increase in η and decrease in h as compared to Shape A, the baseline case. These improvements combined yield an about 20 percent to 30 percent reduction in heat transfer or thermal load on the film protected surface. Shape B, with forward diffusion only, shows a much less significant change in both film effectiveness and overall heat transfer reduction than Shape C

A Numerical Study of Flow and Heat Transfer in a Smooth and Ribbed U-Duct With and Without Rotation

Computations were performed to study the three-dimensional flow and heat transfer in a U-shaped duct of square cross section under rotating and non-rotating conditions. The parameters investigated were two rotation numbers (0, 0.24) and smooth versus ribbed walls at a Reynolds number of 25,000, a density ratio of 0. 13, and an inlet Mach number of 0.05. Results are presented for streamlines, velocity vector fields, and contours of Mach number, pressure, temperature, and Nusselt numbers

Porous CuO–ZnO nanocomposite for sensing electrode of high-temperature CO solid-state electrochemical sensor

A highly porous and nanostructured CuO-ZnO composite has been synthesized for the sensing electrode of a solid-state electrochemical sensor for the high-temperature detection of carbon monoxide. The sensing electrode is made of ZnO nanotetrapod supported CuO nanoparticles. The ZnO nanotetrapods form a three-dimensional interconnected network, leading to a highly porous electrode. The ZnO nanotetrapods on which the CuO nanoparticles are highly dispersedly supported have a high surface-to-volume ratio while maintaining thermal stability at high temperature. Our approach provides an inexpensive route for large-scale production of porous and nanostructured electrodes, which increases the sensitivity of solid-state electrochemical sensors for on-line gas detection at high temperature.

The performance of PEM fuel cells fed with oxygen through the free-convection mode

The feasibility and restrictions of feeding oxygen to a PEM fuel cell through free-convection mass transfer were studied through theoretical analysis and experimental testing. It was understood through the theoretical analysis that the free-convection mass-transfer coefficient strongly depends on the difference in mass fraction or concentration of oxygen at the cathode surface and in the quiescent air. Thus, the mass-transfer rate has a strong dependence on the oxygen concentration at the cathode surface, which can be viewed in terms of the relationship of the fuel cell current density and the fuel cell voltage. Through this analysis, the maximum applicable current density was derived, beyond which there will be an abrupt drop in the output voltage, which results in excessively low power in the fuel cell. Experimental tests were conducted for one PEM fuel cell stack and two single PEM fuel cell units. An excessive drop in output voltage was observed when the free-convection mass-transfer mode was utilized. It was also found that the orientation of the cathode surface affects the performance of the fuel cell, which is mainly due to the fact that the free-convection mass-transfer coefficient depends on the orientation of the involved mass-transfer surface, which is analogous to free-convection heat transfer.

A numerical model coupling the heat and gas species' transport processes in a tubular SOFC

A numerical model is presented in this work that computes the interdependent fields of flow, temperature, and mass fractions in a single tubular solid oxide fuel cell (SOFC). Fuel gas from a pre-reformer is considered to contain H 2 , CO, CO 2 , H 2 O (vapor), and CH 4 , so reforming and shift reactions in the cell are incorporated. The model uses mixture gas properties of the fuel and oxidant that are functions of the numerically obtained local temperature, pressure, and species concentrations, which are both interdependent and related to the chemical and electrochemical reactions. A discretized network circuit of a tubular SOFC was adopted to account for the Ohmic losses and Joule heating from the current passing around the circumference of the cell to the interconnect. In the iterative computation, local electrochemical parameters were simultaneously calculated based on the local parameters of pressure, temperature, and concentration of the species. Upon convergence of the computation, both local details and the overall performance of the fuel cell are obtained. These numerical results are important in order to better understand the operation of SOFCs.

Measurements Over a Film-Cooled, Contoured Endwall With Various Coolant Injection Rates

This paper presents the results of a study of film coverage for coolant injection through an axisymmetric, contoured endwall of a high-pressure turbine first stage vane row. Tests are done on a low speed, linear cascade. The injection is either through a single slot upstream of the leading edges of the vanes or through two slots, one upstream of the other. Because the contouring begins upstream of the leading edges, injection is in an accelerating flow region. The effects of such injection on the secondary flows within the vane cascade are inferred by means of contours of dimensionless temperature. These thermal measurements are made by slightly heating the injection stream above the main flow temperature and documenting the temperatures inside the coolant-mainstream mixing zone. The thermal results are complemented with three-component, hot-wire measurements taken near the exit plane. Performance with different injection rates is discussed. The secondary flow seems to affect the cooling flow strongly when the momentum of the injected flow is small, compared to the main flow momentum. As a result, coolant coverage is non-uniform, with most of the coolant accumulating near the suction side of the passage. As the injection momentum is increased, some pressure-side accumulation of coolant is observed. However, non-uniformity still exists, with a lesser amount of coolant in the central region and more near the suction and pressure surfaces. For the same ratio of coolant to mainstream mass flow rates, cooling through a single slot seems to give more cooling towards the pressure side than does cooling through two slots. With the same mass flow rate, the one-slot case has higher injection momentum than does the two-slot case. This indicates that momentum flux is an important parameter in establishing the distribution of the coolant within the passage.Copyright

Heat Transfer near Turbine Nozzle Endwall

Abstract: This paper gives an overview and reviews recent findings concerning turbine endwall cooling in the literature. The text below begins with a brief discussion of the secondary flows and heat transfer around cascade endwall. This will be followed by a review of recent developments in cooling concepts and related heat transfer results. Key topics include: film cooling, upstream bleeding, endwall contouring, and leakage through component interfaces.

Rheological behavior of clay-nanoparticle hybrid-added bentonite suspensions: specific role of hybrid additives on the gelation of clay-based fluids.

Two different types of clay nanoparticle hybrid, iron oxide nanoparticle clay hybrid (ICH) and Al(2)O(3)-SiO(2) nanoparticle clay hybrid (ASCH), were synthesized and their effects on the rheological properties of aqueous bentonite fluids in steady state and dynamic state were explored. When ICH particles were added, bentonite particles in the fluid cross-link to form relatively well-oriented porous structure. This is attributed to the development of positively charged edge surfaces in ICH that leads to strengthening of the gel structure of the bentonite susensions. The role of ASCH particles on the interparticle association of the bentonite fluids is different from that of ICH and sensitive to pH. As pH of ASCH-added bentonite suspensions increased, the viscosity, yield stress, storage modulus, and flow stress decreased. In contrast, at low pH, the clay suspensions containing ASCH additives were coagulated and their rheological properties become close to those of ICH added bentonite fluids. A correlation between the net surface charge of the hybrid additives and the rheological properties of the fluids indicates that the embedded nanoparticles within the interlayer space control the variable charge of the edge surfaces of the platelets and determine the particles association behavior of the clay fluids.

Trailing-Edge Cooling for Gas Turbines

The trailing-edge section of modern high-pressure turbine airfoils is an area that requires a high degree of attention from turbine performance and durability standpoints. Aerodynamic loss near the trailing edge includes expansion waves, normal shocks, and wake shedding. Thermal issues associated with trailing edge are also very complex and challenging. To maintain effective cooling ensuring metal temperature below design limit is particularly difficult, as it needs to be implemented in a relatively small area of the airfoil. To date, little effort has been devoted to advancing the fundamental understanding of the thermal characteristics in airfoil trailing-edge regions. Described in this paper are the procedures leading to closed-form, analytical solutions for temperature profile for four most representative trailing-edge configurations. The configurations studied are 1) solid wedge shape without discharge, 2) wedge with slot discharge, 3) wedge with discrete-hole discharge, and 4) wedge with pressure-side cutback slot discharge. Comparison among these four cases is made primarily in the context of airfoil metal temperature and resulting cooling effectiveness. Further discussed in the paper are the overall and detail design parameters for preferred trailing-edge cooling configurations as they affect turbine airfoil performance and durability. Also described in this treatment is a current experimental investigation of heat transfer over a trailing-edge configuration. The trailing edge is preceded with an internal cooling channel of pedestal array. The pedestal array consists of both circular pin fins and oblong shaped features or exit teardrops. Downstream to the pedestal array, the trailing edge exits in a pressure side cutback partitioned by the oblong-shaped teardrops. The local heat-transfer coefficient over the entire wetted surface in the internal cooling chamber has been determined using a “hybrid” measurement technique based on transient liquid crystal imaging. The hybrid technique employs the transient conduction model in a semi-infinite solid for resolving the heat-transfer coefficient on the end-wall surface uncovered by the pedestals. The heat-transfer coefficient over a pedestal can be resolved by the lumped capacitance method with an assumption of low Biot number. The overall heat transfer for both the pedestals and end-walls combined shows a significant enhancement compared to the case with thermally developed smooth channel. Near the most downstream section of the suction side, the land, caused by pressure side cutback, is exposed to the stream mixed with hot gas and discharged coolant. Both the adiabatic effectiveness and heat-transfer coefficient on the land section are characterized by using this hybrid liquid-crystal technique. Dr. Cunha is a Principal Engineer at Pratt & Whitney responsible for advanced turbine designs of commercial and military engine programs. Dr. Cunha is also responsible for establishing technical direction for a consortium of Universities as well as in-house research programs in the area of state-of-the-art cooling technologies suitable for use with extremely aggressive gas turbine rotor inlet temperatures. Dr. Cunha’s professional career spans a period of 27 years with turbine original equipment manufacturers including General Electric and Siemens. Dr. Cunha has authored numerous technical papers and gas turbine design standards. Dr. Cunha holds 21 U.S. patents in turbomachinery and is presently a member of the IGTI-ASME heat transfer committee.