High Temperature Thermal Applications Research Articles

High-temperature oxidation of Haynes 282 microtubes in CO2

ABSTRACT The oxidation characteristics of Haynes 282 microtubes measuring 1 mm in diameter and 200 µm thick exposed to CO2 at elevated temperature are reported. After 215 hours at 700°C and 1 atm, a thin, protective chromia scale accumulates up to 2.03 µm from the outer surface, approximately 1% of the thickness, and internal aluminum oxidation is observed 2.59 µm from the inner surface. The results suggest that oxidation is limited by oxygen diffusion through the growing oxide scale layer. The average grain size in the microtube is 10 µm, and it decreases to 5 µm closer to the inner surface. Severe plastic deformation and variable dynamic recovery and recrystallization from the manufacturing process were observed throughout the microtube. Overall grain size and distribution in the microtube were found to be independent of thermal exposure, despite the occurrence of recrystallization. The superalloy microtube geometry shows promising performance in high-temperature thermal applications.

Experimental study on operating characteristics of nitrate salt-based latent heat thermal energy storage unit

Inorganic salts are potential phase-change materials for medium- and high-temperature thermal applications. It is essential to acquire knowledge of their behavior in latent heat thermal energy storage for the design of storage devices and the construction of an energy conversion and utilization system. In this study, a eutectic mixture of NaNO3 (mass ratio of 46%) and KNO3 was selected as phase change material, and a nitrate salt-based latent heat thermal energy storage unit was built to experimentally investigate its operating characteristics during charging and discharging. The thermophysical properties of the eutectic nitrate salt were measured and presented. The air outlet temperature remained almost unchanged during the melting of salt in the charging process, but it decreased gradually during the discharging process because the thermal resistance increased with the salt solidification. The melting time was shortened by 31.0% and 38.1% when the air inlet temperature was increased from 260 °C to 270 and 280 °C, respectively. The solidification time was shortened by 22.2% and 33.3% when the air inlet temperature was reduced from 210 °C to 200 and 190 °C, respectively. When the air mass flow rate was increased from 0.964 g/s to 1.446 and 1.962 g/s, the melting time was shortened by 32.4% and 57.4%, respectively, while the solidification was only shortened by 8% and 16%, respectively. The charging ratio or discharging ratio can be calculated through heat-loss evaluation to depict the thermal energy change in a charging or discharging process. The results indicated that charging ratio increased almost linearly with the melting of the salt. The air mass flow rate had a significant impact on this parameter. The influence of the air inlet temperature was gradually weakened with the increasing air inlet temperature. Owing to the coupling effect of heat loss and airflow, the influence of air parameters on the discharging ratio was weak. The effectiveness of storage indicates the extent to which the latent heat of salt can be utilized. It increased from 29.7% to 52.8% when the air inlet temperature was reduced from 210 to 190 °C. This study provides insights into the phase-change characteristics of the nitrate salt and the nitrate salt-based latent heat thermal energy storage unit.

Thermal selective coatings and its enhancement characteristics for efficient power generation through parabolic trough collector (PTC)

World climate is an area of concern due to the use of fossil fuels that have been the most commonly preferred resource of energy since the industrial revolution and urbanization. The target to maintain the lowest level of carbon emissions and greenhouse gases has created an urge to look for renewable energy resources. Among the renewable energy resources available worldwide, solar energy is considered as one of the feasible and mature technologies in view of large-scale commercial deployment. Solar photovoltaic and solar thermal conversion (STC) techniques have been implemented so far and are still advancing towards cost-effective solutions. Parabolic trough collector (PTC) is one such economical and feasible STC technology as far as high-temperature thermal applications are concerned and are being widely used for power generation. This paper is an attempt to present the current scenario of PTC technology along with its various advancements over the years. Further in this paper, selective coatings, coating techniques and heat collection element (HCE) or receiver are discussed in detail with regard to their advancements. The present work also illustrates the progressive trends in PTC technology, particularly with respect to various heat transfer fluids, HCE inserts, selective coatings and other performance factors along with some futuristic aspects with respect to coatings and receiver inserts in view of high thermal performance.

Performance Evaluation of a Forward Curved Blower for Thermal Applications

This paper presents the performance evaluation of a forward curved blower, for air supply at high temperature thermal applications such as incineration and biomass gasification. For such an application to be successful, and work as intended, the blower and system must be compatible both structurally as well as from a performance standpoint. Tests were carried out in order to determine the performance of the designed blower under actual working conditions. Agricultural wastes such as: groundnut shell, rice husk and bagasse were used to carry out these tests in an existing incinerator. A constant mass of 20 kg each was measured for the variables and fed into the incinerator. The blower was operated at rotational speeds and air velocities of 3203 rpm, 3111rpm, 3078 rpm and 24.4m/s, 23.8m/s, 21.3m/s as measured using a tachometer and anemometer respectively. Temperatures were recorded using two digital thermocouples at 300 seconds intervals. The obtained data were varied at nine levels and laid in randomized complete design (RCD) which was replicated three times for a total of 243 experimental treatments. The Design Expert software version 7.0 was used to analyze and interpret the experimental data. A peak temperature of 891°C was recorded at 3111 rpm and an air velocity of 23.8 m/s. Major characteristics of the blower such as the power output and mechanical efficiency were obtained as 0.56 kW and 62% respectively. The designed blower is suitable for gasification operations which require temperatures of about 750°C.