Christopher Yap

Synthesis of mesoporous titanium dioxide by soft template based approach: characterization and application in dye-sensitized solar cells

The present work demonstrates the usefulness of the soft-template directed approach in preparing mesoporous titanium dioxide and applying it in dye-sensitized solar cells (DSSCs). Mesoporous titanium dioxide with nanograins of dimension in the range, 16–20 nm were prepared through the soft-templating approach using various cationic surfactants such as octyl-, dodecyl-, cetyl trimethylammonium bromide with different surfactant compositions and titania precursor concentrations. As-synthesized mesoporous titanium dioxide samples were characterized by TGA, PXRD, FESEM, HRTEM and surface area measurements, used as photo-electrode material in DSSCs. Under global AM 1.5 solar irradiation, the best photovoltaic performance of 7.5% with a short circuit photocurrent density of 14.2 mA/cm−2, an open circuit voltage of 748 mV, and a fill factor of 70.83% were obtained for the DSSC using a film of mesoporous TiO2 synthesized from the cetyl trimethylammoniumbromide surfactant. In contrast, the use of commercial titania powder P25 resulted in an efficiency of 5.0% under similar conditions. The structural and morphological merits of these template-directed materials will be especially addressed in comparison with their traditional bulk forms.

A general model for studying effects of interface layers on thermoelectric devices performance

Abstract Interface layers play important roles in thermoelectric (TE) devices. The present study employs a phenomenological model to study the effects of internal and/or external interface layers on TE devices performance. Sets of general performance formulae are derived for both TE coolers and generators. Some simplifications are presented, and familiar performance formulae are found by introducing effective or equivalent properties such as electrical resistivity, thermal conductivity and Seebeck coefficient. Moreover, temperature–entropy diagrams are built to understand the effects of interface layers on TE thermodynamic cycles. Together with the power-efficiency curves, TE devices performance can be fully comprehended. In addition, a popular example is analysed, and new results are presented.

The maximum temperature difference and polar characteristic of two-stage thermoelectric coolers

The maximum temperature differences (MTDs) of three types of two-stage thermoelectric (TE) coolers are analysed, which are the ones with thermocouples at two stages electrically connected in series and parallel and electrically separated, respectively. The analyses are all performed with respect to an important ratio r in two-stage TE coolers, viz the ratio of thermocouple number between stages. The MTDs of the first and the last configurations can be both far higher than that of a one-stage TE cooler, which may promote the cryogenic applications of TE coolers. When r approaches to infinity, close-form formulae of their limiting MTDs are found. In the mean time, the polar characteristic of two-stage TE coolers is discussed, when the supplied electric current(s) is alternated.

Numerical Analysis of Blockage and Optimization of Heat Transfer Performance of Fractal-like Microchannel Nets

The conjugate fluid flow and heat transfer characteristics of fractal-like microchannel nets embedded in a disk-shape heat sink are investigated using a three-dimensional computational fluid dynamics (CFD) approach. A constant heat flux is applied to the top wall of the heat sink. The intrinsic advantages of fractal-like microchannel nets such as low flow resistance, temperature uniformity, and reduced danger of blockage compared with the traditional parallel channel nets are demonstrated. In addition, various optimized designs with parameters such as the number of branches, number of branching levels, and number of channels that reach the center of the disk are addressed in this context.

Optimization of two-stage thermoelectric coolers with two design configurations

Abstract This article presents the optimization of two-stage thermoelectric (TE) coolers arranged in two practical design configurations, and the optimization methods can be extended to other multi-stage designs. The first arrangement is the conventional pyramid-styled multi-stage TE cooler, in which the supplied electric current is uni-directional with the top stage of the unit being the coldest. A second design configuration comprises the cuboid-styled multi-stage cooler, in which the supplied current can be alternated, and thus, the top and bottom stages can be switched between heating and cooling modes. Optimizations for the two-stage TE coolers are performed at both its two optima, namely, the point of maximum cooling capacity and optimal coefficient of performance, or COP for short. In the first design of the TE cooler, the optimization goal is geared towards determining the optimum ratio of the number of TE modules between stages when the total number of modules is kept constant, while in the second design configuration, it is to solve for the optimum ratio of electric current between stages. Optimum design parameters for the two types of two-stage TE coolers are compared on given conditions. For practical purposes, all properties of TEs are based on commercial TE materials.

Maximum mass flow ratio due to secondary flow choking in an ejector refrigeration system

Abstract The occurrence of flow choking in an ejector of an ejector refrigeration system (ERS) was analysed and a model for predicting the maximum flow ratio of the ejector was developed. The multi-parameter equation to calculate the mass flow ratio takes into account the performance of the primary nozzle, the flow entrainment and mixing relating to ejector geometry and operating conditions. We validated the model using the reported experimental data of refrigerant R113, R141b and steam ERS. The present model was shown to provide better accuracy compared with results obtained by applying the existing 1-D ejector theory. We discussed the application of the model and highlighted the significance of the parameters for future work.

Effect of bifurcation angle in tree-shaped microchannel networks

Tree-shaped microchannel networks are being considered for thermal designs that require high heat transfer densities for the cooling of modern electronics. Here we investigate the effect of the bifurcation angles in the constructal nets on the fluid flow and heat transfer characteristics of such networks using a three-dimensional computational fluid dynamics approach. Results show that the bifurcation angle is an important factor which determines the performance of such cooling nets. Surface temperature distributions and pressure drop along the flow paths are analyzed and compared. For the same boundary conditions, a lower temperature and pressure variation is observed at lower bifurcation angles.

Heat Transfer, Temperature, and Velocity Measurements Downstream of an Abrupt Expansion in a Circular Tube at a Uniform Wall Temperature

Detailed heat transfer, temperature, and velocity data are reported for the turbulent flow downstream of an abrupt increase in tube diameter (2.5:1) in which the downstream tube is maintained at a uniform elevated temperature. The heat transfer experiments cover downstream Reynolds numbers ranging from 4,300 to 44,500, the flow being fully developed at the exit of the small tube (i.e., the abrupt expansion step). Maximum local heat transfer coefficients are proportional to the upstream Reynolds number to the power 2/3 with the location of the maximum moving upstream slightly as the Reynolds number is increased. Heat transfer data at a Reynolds number of 17,300 are supplemented by velocity and temperature profiles, which are especially informative. They bring out clearly that the viscous sublayer, despite the thinning that is believed to occur in the vicinity of the reattachment point, still provides the major resistance to heat transfer. The correct prediction of the Nusselt-Reynolds number relation in reattaching flows is thus crucially dependent on the variation of turbulent diffusivity in the buffer region of the flow.

Pool Boiling Heat Transfer of Water on Finned Surfaces at Near Vacuum Pressures

This research paper presents a study of boiling heat transfer from longitudinal rectangular-finned surfaces immersed in saturated water at low vapor pressures. Finned surfaces with assorted fin spacing, fin thicknesses, and fin heights on a copper based surface have been investigated. All the finned surfaces were found to increase both boiling heat transfer coefficients and critical heat fluxes. An optimal fin thickness was found for a configuration, and heat transfer coefficients have been obtained at the pressures. Factors affecting the boiling characteristics have been identified and the optimal enhancement requires a balance of the active nucleation sites, bubble flow resistance, natural convection, thin film evaporation, liquid superheating, heat transfer area, bubble coalescence, and liquid reflux resistance. High speed visualization of vapor plug and vapor film generation on the boiling surfaces has revealed significant insights into the boiling mechanisms at low saturation pressures.

Laminar Heat Transfer in Constructal Microchannel Networks With Loops

Heat sinks with radial and constructal branching microchannel networks with loops are examined numerically. Radial and constructal networks are embedded in disk-shaped heat sinks. Constructal nets with loops are found to be more robust than the radial ones, when one or more channel segments are blocked. Since complex constructal networks would involve problems in manufacturing, constructal channel nets with loops may be a better choice in engineering applications. Networks with loops and without loops are compared. Results show that the constructal nets with loops provide a great advantage when the structure experiences accidental damage in one or more subchannel segments, since the loop assures the continuity of flow. In spite of blockage, the performance of the network has only a small drop considering the increased pressure drop.