Huann-Ming Chou

Heat transfer characteristics of an insulated regular polygonal pipe by using a wedge thermal resistance model

The relation of the critical and neutral thicknesses and heat transfer characteristics for an insulated regular polygonal pipe are investigated by using a new wedge thermal resistance model based on its varying pipe surface area. It is found that the dimensionless heat transfer characteristics of the insulated polygonal pipes are the same as those of an insulated circular pipe. It is also found that the conventional constant surface area plate model cannot be used to analyse the insulated regular polygonal pipes, especially at the situations of small to medium pipe sizes and/or with a greater insulation thickness.

Elasto-optics in double-coated optical fibers induced by axial strain and hydrostatic pressure.

Stresses, microbending loss, and refractive-index changes induced simultaneously by axial strain and hydrostatic pressure in double-coated optical fibers are analyzed. The lateral pressure and normal stresses in the optical fiber, primary coating, and secondary coating are derived. Also presented are the microbending loss and refractive-index changes in the glass fiber. The normal stresses are affected by axial strain, hydrostatic pressure, material properties, and thickness of the primary and secondary coatings. It is found that microbending loss decreases with increasing thickness, the Youngs modulus, and the Poissons ratio of the secondary coating but increases with the increasing Youngs modulus and Poissons ratio of the primary coating. Similarly, changes in refractive index in the glass fiber decrease with the increasing Youngs modulus and Poissons ratio of the secondary coating but increase with the increasing Youngs modulus and Poissons ratio of the primary coating. Therefore, to minimize microbending loss induced simultaneously by axial strain and hydrostatic pressure in the glass fiber, the polymeric coatings should be suitably selected. An optimal design procedure is also indicated.

Heat transfer characteristics of an insulated regular polyhedron by using a regular polygon top solid wedge thermal resistance model

The relation of the critical and the neutral thickness and heat transfer characteristics for an insulated regular polyhedron are investigated by using a new regular polygon top solid wedge thermal resistance (RPSWT) model obtained from a solid angle concept. It is found that the dimensionless heat transfer characteristics of the insulated polyhedrons are the same as those of an insulated sphere even though their actual heat transfer rates are different. It is also found that the conventional constant surface area plate model cannot be used to analyze the insulated polyhedrons, especially in situations of small to medium body size and/or with a greater insulation thickness. The single RPSWT model should also be applied to the heat transfer problem of the wedge block with a regular polygon top.

Reaction of the electrically-heated catalyst of a four-stroke motorcycle engine under cold-start conditions with additional enrichment of the intake mixture

Abstract By using additional fuel enrichment, the influence of an electrical heater and the oxidation of unburnt exhaust gases on the reaction of an electrically-heated catalyst under cold-start condition of a four-stroke motorcycle engine was investigated. The effects of different cold-start enriching times, heating temperatures, exhaust settings of the CO level and heating positions were studied. The heating temperatures were 60, 80, 100 and 120°C, and the CO levels were 1.0 and 1.8 per cent. The two heating positions were at the inlet and the mid-section of the catalyst. The results showed that additional enrichment of the intake mixture can reduce the heating temperature for self-reaction of the catalyst as well as improve the conversion effciency of the catalyst on the exhaust emissions. High conversion effciency has been achieved for a heating temperature as high as 100°C for the 1.8%CO setting level. It has also been revealed that self-reaction of the catalyst was not possible for a heating temperature of up to 140°C in the case of a 1.0%CO setting level.

Optimum interior area thermal resistance model to analyze the heat transfer characteristics of an insulated pipe with arbitrary shape

The heat transfer characteristics for an insulated regular polygonal (or circular) pipe are investigated by using a wedge thermal resistance model as well as the interior area thermal resistance model Rth=t/Ks/[(1−α)A2+αA3] with a surface area weighting factor α. The errors of the results generated by an interior area model can be obtained by comparing with the exact results generated by a wedge model. Accurate heat transfer rates can be obtained without error at the optimum αopt with the related t/R2. The relation between αopt and t/R2 is αopt=1/ln(1+t/R2)−1/(t/R2). The value of αopt is greater than zero and less than 0.5 and is independent of pipe size R2/Rcr but strongly dependent on the insulation thickness t/R2. The interior area model using the optimum value αopt with the related t/R2 should also be applied to an insulated pipe with arbitrary shape within a very small amount of error for the results of heat transfer rates. The parameter R2 conservatively corresponds to the outside radius of the maximum inside tangent circular pipe within the arbitrary shaped pipes. The approximate dimensionless critical thickness tcr/R2 and neutral thickness te/R2 of an insulated pipe with arbitrary shape are also obtained. The accuracies of the value of tcr/R2 as well as te/R2 are strongly dependent on the shape of the insulated small pipe. The closer the shape of an insulated pipe is to a regular polygonal or circular pipe, the more reliable will the values of tcr/R2 as well as te/R2 be.

Heat transfer characteristics of an insulated regular cubic box by using a solid wedge thermal resistance model

Abstract A new solid wedge thermal resistance model is developed in this investigation and used to solve the relation of the critical and the neutral thickness and heat transfer characteristics for an insulated regular cubic box based on its varying surface area. It is found that the dimensionless heat transfer characteristics of the insulated cubic box are the same as those of an insulated sphere. It is also found that the conventional constant surface area plate model cannot be used to analyse the insulated cubic box, especially at the situations of small to medium box sizes and or with a greater insulation thickness. The solid wedge thermal resistance model should also be applied to the heat transfer problem of the block having a shape similar to a solid wedge.

Effects of Temperature-Dependent Viscosity on Natural Convection in Porous Media

This analysis has studied natural convection for the temperature-dependent viscosity of fluids inside porous media between two concentric spheres by numerically solving the Brinkman–Darcy–Forchheimer model, vorticity transport, and energy equations. Parameters included Rayleigh numbers (5.0 × 103–8.0 × 104) at radius ratios of 1.5, 2.0, and 3.5 with porosities of 0.4 and 0.9 for variable-viscosity fluids with Prandtl numbers (158, 405, and 720) when the Darcy number was changed at 0.1 and 0.001. The results showed that the mean Nusselt number varied with Rayleigh number, porosity, radius ratio, and variable viscosity but did not change with the Darcy number.

The effects of vibration and reciprocating on boiling heat transfer in cylindrical container

Abstract The performance of enhancing boiling heat transfer in cylindrical container was studied experimentally. The effects of the heat load, the grain size of the steel and plastic balls added in the container, the vibration frequency of the disk, and the reciprocation frequency of the cylindrical container on the characteristics of heat transfer were investigated in detail for water as working fluid. The experiment results show that the active methods can really improve the boiling heat transfer promisingly. That is, the heat transfer can be raised only by adding an appropriate amount of additives inside the heated container with vibration or reciprocating rotation, then the boiling heat transfer can be raised by 20∼65% without increasing the heating area.

Simultaneous Estimation of Boundary Heat Flux and Convective Heat Transfer Coefficient of a Curved Plate Subjected to a Slot Liquid Jet Impingement Cooling

In this study, an inverse algorithm based on the conjugate gradient method and the discrepancy principle is applied to simultaneously estimate the unknown boundary heat flux and convective heat transfer coefficient in a curved plate cooled by an impinging slot jet from the knowledge of temperature measurements taken within the plate. Subsequently, the distributions of temperature in the plate can be determined. It is assumed that no prior information is available on the functional forms of the heat flux and convective heat transfer coefficient; hence the procedure is classified as the function estimation in inverse calculation. The temperature data obtained from the direct problem are used to simulate the temperature measurements, and the effect of the errors and locations in these measurements upon the precision of the estimated results is also considered. Results show that an excellent estimation on the heat flux and convective heat transfer coefficient and temperature distributions can be obtained for the two test cases considered in this study.

The effect of vibration on rotation on boiling heat transfer in heating tank with different grooved pattern on its base and copper balls enclosed inside

The effect of either vertical vibration or reciprocating rotation on boiling heat transfer in cylindrical heating tank with different grooved pattern on its base and with copper balls enclosed inside was studied experimentally. The experimental model consists of seven aluminum cylindrical tanks with different grooved pattern on their base and copper balls enclosed in the heating section and a vertical vibration disc or a reciprocating rotation disc attached to the bottom of the cylindrical tank base. By utilizing the vibrating and rotating test on the heating tank base with different grooved pattern, the effect on boiling heat transfer was investigated. The working fluid is distilled water. Besides the effect of different grooved pattern in heating tank, different frequency of vertical vibration and reciprocating rotation on boiling heat transfer was studied, the effects of the diameter and quantity of enclosed copper balls on boiling heat transfer was also investigated in this experiment. The result shows that the boiling heat transfer was reduced and converged to a constant as the frequency of reciprocating rotating disc increases after copper balls were enclosed inside the heating tank. There is an optimum frequency of vertical vibrating disc for boiling heat transfer. 15% increment of boiling heat transfer can be obtained with proper grooved pattern on aluminum heating tank base and with proper volume of copper balls enclosed inside aluminum cylindrical tank.