Tomohiro Honda

A NUMERICAL ANALYSIS OF LAMINAR FREE CONVECTION AROUND AN ISOTHERMAL SPHERE: FINITE-DIFFERENCE SOLUTION OF THE FULL NAVIER-STOKES AND ENERGY EQUATIONS BETWEEN CONCENTRIC SPHERES

A numerical solution of transient laminar free convection between two concentric spheres is presented for Pr= 0.7 and Ra =100. The steady average heat transfer coefficient for the inner sphere becomes independent of the radius of the outer sphere when it is more than 40 times that of the inner sphere, and independent of the number of grid points when they are at least 61 and 46 for the radial and azi-muthal directions, respectively. Steady state is reached at a dimensionless time τ = 6 for the average heat transfer coefficient and at τ = 12 for the convection flow. The steady local Nusselt number near the top and bottom stagnation points is different from that of the previous solution of the boundary-layer equations containing curvature terms, while the average Nusselt numbers of both solutions agree within 2%.

Experiments on Flow Boiling Heat Transfer of Almost Pure CO2 and CO2-Oil Mixtures in Horizontal Smooth and Microfin Tubes

Experiments on the flow boiling heat transfer of almost pure CO2 and CO2-oil mixtures in horizontal smooth and microfin tubes have been carried out. The smooth tube is a stainless steel tube with an inside diameter of 3 mm, and the microfin tube is a copper tube with a mean inside diameter of 3.04 mm. Experiments were carried out at mass velocities from 190 to 1300 kg/(m2·s), a saturation temperature of 10°C, heat fluxes from 5 to 30 kW/m2, and an oil (PAG) circulation ratio from <0.01 to 0.72 wt%. In the case of almost pure CO2, the flow boiling heat transfer is dominated by nucleate boiling for both smooth and microfin tubes. In the case of CO2-oil mixtures, the heat transfer coefficients are much lower than those of almost pure CO2. The strong dependence on heat flux disappeared due to the existence of oil. This is due to nucleate boiling being suppressed by the oil film. The dryout quality decreases greatly with an increase in mass velocity for the smooth tube and is little affected by mass velocity for the microfin tube. The effect of oil on dryout quality is very small in both tubes.

A NUMERICAL ANALYSIS OF LAMINAR FREE CONVECTION AROUND AN ISOTHERMAL SPHERE

A numerical analysis is carried out about the thick boundary layer of steady laminar free convection around an isothermal sphere by a method quite similar to that of the previous study for a horizontal cylinder. The results for the average heat transfer coefficient for Pr = 0.7, 10, and 100 and Ra = 10-2, 102, and 106 are expressed within an accuracy of 1% as follows: Num = 2 + 0.S07C(Pr) Ram and C(Pr) is a function of Pr. This expression predicts values a little lower than experimental results in the moderate Rayleigh number range. Also discussed are the radial distributions of temperature and velocity components and the azimuthal distribution of local Nusselt number, and the results are compared with previous theoretical and experimental results for the average Nusselt number.

Numerical and experimental studies on the measurement of thermal conductivity of a silicone rubber plate as a reference material

Silicone rubber is used as a reference material to examine the accuracy of measuring apparatus. However, the thermal conductivity of silicone rubber has been reported as ranging from 0.20 to 0.25 W/(m·K) at 293.15 K; thus an engineer cannot determine the accuracy of measuring apparatus. In order to obtain the exact value for the thermal conductivity of silicone rubber, the authors have developed a measuring apparatus according to the Guarded Hot Plate method. The dimensions of the test sample were 100 mm x 100 mm x 10 mm. This is sufficient to measure thermal conductivities ranging from 0.18 to 0.26 W/(m·K). The thermal conductivity obtained for a silicone rubber plate in this study was close to 0.25 W/(m·K). It was found that deformation of the silicone rubber has an effect on the thermal conductivity. The accuracy of our thermal conductivity measurement is estimated to be within ±3.5%.

Measurement of the Speed of Sound in 2,3,3,3-Tetrafluoropropylene (R-1234yf) Liquid Phase with an Ultrasonic Sensor

The speeds of sound in near saturated and subcooled liquid phases of R-1234yf have been measured using a ultrasonic sensor. The measurements were conducted at the temperature range from 0 to 80 °C, and the pressure range from 0.316 to 2.52 MPa. In order to validate the ultrasonic sensor, the measurement in pure water at the temperature range from 0 to 80 °C was also performed. The measurement result of water was compared with those calculated from REFPROP (based on IAPWS-95) and PROPATH (based on IAPWS-IF97). The result of water agrees well with that calculated from REFPROP within a maximum deviation of 0.28%, except for that at temperature of 80 °C. And it agrees well also with that calculated from PROPATH within a maximum deviation of ±0.2%. The measurement results for near saturated and subcooled liquid phases of R-1234yf were compared with those calculated from REFPROP and JSRAE thermodynamic table.

Study on Guarded Hot Plate Apparatus for Measurement of Thermal Conductivity of Small Polymer Specimens

The aim of this work is to develop a guarded hot plate test apparatus for measuring the thermal conductivity of polymer specimen, the size of which is smaller and thicker than that recommended by the ISO and JIS. The authors made three experimental apparatus suitable for the different sizes of specimen 50×50, 100×100 and 200×200 mm2 in area, measured the thermal conductivity of polymer as homogeneous materials, which ranged from 0.2 to 0.6 W·m−1 ·K−1 , and carried out the numerical analysis. The uncertainty for the measurement of the thermal conductivity of the polymer specimens is estimated to be within ±4% using our test apparatus. For the test apparatus suitable for the specimen 50×50 mm2 , the gap has an effect on the measurement of the thermal conductivity. In addition, the temperature difference ΔTg of the heat source has a direct influence on the deviation in the thermal conductivity data. The influence of the aspect ratio of the specimen and gap width on the measurement is estimated to be within 0.9% for the specimens 50×50 mm2 used in the present study.Copyright