Fiber Bragg Gratings for distributed cryogenic temperature measurement in a tube in tube helically coiled heat exchanger

A novel technique to measure the cryogenic temperature distribution of a helically coiled Tube-in-Tube (TIT) heat exchanger using Fiber Bragg Grating (FBG) array has been reported. The measured temperature distribution will help in estimating the heat exchanger efficiency and will be used to investigate its performance. The measurement of the temperature distribution has previously only been simulated, as using a standard temperature sensor would inadvertently hamper the flow. This issue can be solved by using a small sized FBG sensor array. In this paper, the principle, design and installation of sensors are explained and the initial experiments are carried out to demonstrate the measurement technique. The experimentally measured temperature distribution was found to be agreeable with the simulated values with minor deviations.

I. Introduction

Recuperative heat exchangers are used extensively for low temperature applications, especially in Joule Thomson and Brayton cryocoolers. High effectiveness and compactness are essential characteristics for such heat exchangers. Frame and Coles [1] were among the first to suggest the use of a Tube-in-Tube (TIT) helically coiled heat exchanger for its higher heat transfer rate and compact nature. Heat exchangers have been found to play a crucial role in determining the performance of cryocoolers [2]. TIT heat exchanger can be numerically modelled in order to study its fluid flow and heat transfer characteristics. The predictive models that have been developed [3], [4] have a difficulty in defining the parameters that critically influence heat transfer characteristics. As a result, they can only give a gist of the heat transfer in a TIT heat exchanger, but not the precise temperature distribution or the heat transfer characteristics. Many researchers have also attempted to experimentally study the TIT heat exchanger [4], [5] to understand the heat transfer and fluid flow. However, the exact temperature distribution was not measured in any of these studies. Measuring the actual temperature distribution in a TIT heat exchanger can help in improving the heat exchanger design and consequently, its efficiency. Hence, in order to get the temperature profile, it is essential to measure it experimentally. The standard temperature sensor like the resistance temperature detector (e.g. PT 100) cannot be placed inside the TIT heat exchanger without obstructing the flow through it [6]. An attempt to integrate such sensors may affect the performance of the TIT heat exchanger. This brings to the fore a need for a small sized sensor that can be used to measure cryogenic temperatures. Previously Fiber Bragg Grating (FBG) sensors have been used to evaluate the temperature distribution of multilayer insulation [7]. In the present study, Bragg gratings fabricated at different locations in a single fiber is used for temperature measurement inside the TIT heat exchanger.