Infrared-Thermography and numerical investigation of conjugate heat transfer in Roots blower

In Positive displacement machines, the operating temperature of the gas is different than the rotor and casing temperatures, causing heat to flow (heat transfer) from gas to rotors and casing and vice versa. Thermal expansion of the machine components which occurs at high speed, changes operating clearances between rotors and the casing. This leads to the deterioration of the efficiency and reliability of the machine. The present research work focuses on both, the experimental study of heat transfer at the rotor surface during the operation of the machine and the development and validation of CFD models. The heat transfer phenomenon between solid and fluid is treated in the numerical model as conjugate heat transfer (CHT).

 

An experimental study on an optically accessible roots blower is performed using a high-speed infrared thermography system. Required optical access on the machine is constructed from sapphire glass. A national Instrument-based data acquisition system is designed to achieve phase-lock synchronization of the camera exposure with the rotation angle of the lobe. This system is also extended to measure and control machine operating parameters such as pressure, temperature, flow, power, and speed. A CFD model of the experimental Roots Blower was evaluated with fluid flow and conjugate heat transfer. The additional complexity of the model involved deforming rotor grids and the in-house tool SCORG was used for this purpose. Results of steady-state lobe surface temperature and housing external temperature have been verified in the numerical model at various operating conditions. This CHT model will be used in the future to evaluate design modification and conceptualize methods for reduction in leakage losses and ensure reliable operation of the machine.

 

In summary, this study focuses on the development of the numerical model for CHT and the validation of the model using experimental data. A high-speed infrared thermography test setup is developed to measure lobe surface temperature during running conditions of the machine. Thermograms are successfully captured at three various pressure ratios 1.2, 1.4, and 1.6, and speeds ranging from 1000 RPM to 2000 RPM. This measurement provides quantitative data for the validation of the developed numerical model.