Erling Næss

Influence of the Velocity, Local Position, and Relative Humidity of Moist Air on the Convective Mass Transfer Coefficient in a Rectangular Tunnel — Theory and Experiments

This paper explores the effect of the relative humidity (RH) of moist air as well as velocity on the convective mass transfer coefficients (CMTC) at different positions. Experiments determined the local mass transfer coefficients of three water samples mounted in line with the bottom wind tunnel surface. The experiments were carried out at a temperature of 20°C, relative humidities of 30, 50, and 70%, and velocities between 0.1 and 5m/s. The experimental data show that the convective mass transfer coefficient is a function of velocity, the RH of the air and the local position. The experimental results are compared to expressions for the convective mass coefficients from the literature.

Experimental Study of Thermal Effects in a Hydrogen Cryo-Adsorption Storage System

While the main barrier for the development of hydrogen adsorption type storage systems remains the material development, an improved thermal management may offer solutions to minimize the penalties in the amount of stored gas during fast-filling and the residual amount of hydrogen during discharging operations. The emphasis of this work was to experimentally investigate the dynamical thermal behavior of a hydrogen cryoadsorption storage system during fast-filling operations. The experiments were conducted with granulated activated carbon and MOF adsorbents. The influence of the charge pressure and the gas flow rate on the temperature elevations and the amount of filled hydrogen gas was analyzed. The heat generated in the storage vessel originates from the released heat of adsorption, gas compression work and thermal energy transfer that takes place when high pressure gas at the ambient temperature is introduced to the tank. A typical average temperature increase observed during the charging of the test tank, filled with the activated carbon (NORIT R0.8 extra), to 2 MPa was about 21 K. Such temperature elevation leads to a significant decrease in adsorption storage capacity.Copyright

Influence of Thermal Effects During Fast Filling Operations on Adsorption Capacity in a Hydrogen Cryo-Adsorption Storage Tank

Charging and discharging operations of on-board hydrogen adsorption storage systems involve exothermic and endothermic processes. Temperature elevations caused by the released heat of adsorption, compression work and thermal mass introduced from the inlet gas result with a reduction of the storage capacity. The main objective of this work was the investigation of the impact of temperature elevations on the decrease of adsorption storage capacity during high-pressure charging of a hydrogen cryo-adsorption storage tank. The experimental operating conditions were compatible with practical applications for hydrogen adsorption on-board storage systems. The analysis was conducted with two adsorbent classes: activated carbon (NORIT R0.8) and metal-organic-frameworks (Cu-BTC-1,3,5). Adsorption isotherms for hydrogen uptake for both adsorbents are measured by a gravimetric method and fitted in accordance to the Langmuir equation. The experimental study was carried out in a cylindrical tank with granular adsorbents in which the bed temperature was measured at various positions. A typical average temperature increase in the core of the storage column during hydrogen charging experiments with the CuBTC and NORIT R0.8 was 16.2 K and 20.4 K at 2 MPa respectively. Such temperature elevation results in a loss in the adsorption storage capacity of 14.75% for the system packed with the CuBTC adsorbent. Solutions for increased efficiency of the hydrogen cryo-adsorption storage tanks are proposed.Copyright

Measurement of the convective moisture transfer coefficient from porous building material surfaces applying a wind tunnel method

This article presents results for the average convective moisture transfer coefficients of several porous building material samples exposed to airflow. The experimental measurements explore the effect of the various air velocities, air temperatures and local positions on the average convective moisture transfer coefficients. Selected building materials were soaked in distilled water at least 2 weeks before the measurements. A thin building specimen with moisture content close to the saturation point was mounted in level with the bottom wind tunnel surface. A stable airflow regime was measured over the thin samples placed in the specimen holder. Water from the sample holder was absorbed from the bottom side of the building materials and evaporated from the upper side of the specimen during the airflow exposure. Two different membranes were fixed over the water cup as reference materials for comparison. The measurements were carried out at a relative humidity of 50% ± 3%, air temperatures of 23.6°C ± 0.5°C, 26.5°C ± 0.5°C and 30.0°C ± 0.5°C, and air velocities of 1.1, 3.0 and 5.5 m/s. The experimental data show that the convective moisture transfer coefficient is a function of velocity, temperature difference between the ambient air and material surface, local position as well as of the material type. The experimental results from water surfaces were compared to the expressions for the convective moisture transfer coefficients from the literature.

Pool Boiling Heat Transfer of Dowtherm-A and Dowtherm-J on Smooth and Roughened Vertical Heater Surfaces

Heat transfer coefficients in nucleate pool boiling of the organic heat transfer fluids Dowtherm A and Dowtherm J were obtained experimentally using a vertical electrically heated cylindrical carbon steel surface. The experiments were carried out at system pressures between 0.2–4.9 bar and heat fluxes in the range 20 to 230 kW/m2 and using different surface roughnesses. The heater surface was polished with emery paper 1200 to an average surface roughness (Ra ) of 0.02 and 0.13 μm for Dowtherm A and J respectively, and roughened to an average surface roughness (Ra ) of 2.42 μm by emery paper 40, and by sandblasting to an average (Ra ) of 1.3 and 3.6 μm. The experimental results on the roughened surfaces showed that the heat transfer coefficient increased with increasing pressure, increasing surface roughness and increasing heat flux. A new correlation for predicting the pool boiling heat transfer coefficient for Dowtherm A and Dowtherm J is proposed. The correlation is an adapted Gorenflo [1], [2] correlation, and is valid for pressures between 0.2 and 5 bar, heat fluxes up to 230 kW/m2 and surface roughnesses between 0.02 and 3.6 μm. The accuracy of the correlation is within ±15% compared to experimental data.Copyright