Measurement of boiling heat transfer coefficient of R-134a in a circular tube using fiber optic temperature sensors

This paper investigates the viability of a using a distributed fiber optic sensor to provide high resolution temperature data along the length of a tube-in-tube heat exchanger during evaporation of R-134a in a horizontal, circular tube with a diameter of 4.7 mm. Refrigerant flows in an inner tube surrounded by an annulus containing heated water which provides thermal input to the test section. A fiber optic sensor is used to measure the temperature of the heating fluid in the annulus which enables the local heat duty and boiling heat transfer coefficients to be calculated. Experiments are conducted at saturation temperatures of 15 ? and 20 ?, mass fluxes of 200 to 400 kg m^-2 s^-1. The fiber optic temperature data is verified using co-located resistance temperature detectors (RTDs) immersed in the flow of water, and the calculated heat transfer coefficients are compared to a literature correlation for boiling of R-134a. The fiber optic sensor is advantageous over discrete point temperature measurement devices which are limited by physical size and cost of the sensors. Discrete sensors can also be more invasive to the experiment, such as the axial flow field temperature measurement of interest here. Successful demonstration of this measurement technique is of interest in many fields, especially where temperature data is required over a large distance, through complex geometries, or where high resolution data is required.