Experimental Apparatus for the Study of micro Heat Exchangers with Inlet Temperatures between -200 and 200 °C and Elevated Pressures

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.The current paper presents a test bench for micro-fabricated Recuperative Counter Flow Heat Exchanger (RCFHE). The bench is suitable for up to 200 K difference between inlets temperatures and operating pressures up to 32 MPa. The experimental setup allows controlling the physical state of the gas (i.e. temperature, pressure and flow rate) at the RCFHE inlets. The bench has 5 controlled parameters and 5 more that are monitored and enables studying each of the hot and cold channels separately. We demonstrate a steady supply of liquid nitrogen into the device for 10 minutes without thermal insulation of the specimen. Another run is a steady state experiment with a temperature difference of about 20-30 K between inlets. These show that the apparatus is capable of characterizing heat exchangers and serve as preliminary results

Development of a Miniature Fast Cool Down J-T Cryocooler

Presented at the 16th International Cryocooler Conference, held May 17-20, 2008 in Atlanta, Georgia.One major advantage of Joule-Thomson (J-T) cryocoolers over other cryocoolers is the ability to achieve a very fast cool-down, in the range of only a few seconds. The main fluid is chosen according to the desired cooling temperature and the fast cool-down is usually obtained by allowing high flow rates during this transient process. A primary cooling stage may be added in order to cool the main fluid and reduce the cool-down time, but it has the price of two pressure vessels and a more complex, and bigger, cryocooler. Fast cool-down is usually required when the total cooling time is relatively short, a few seconds up to a few minutes. Thus, fixed-orifice cryocoolers are preferable, according to manufacturing and reliability aspects. However, the flow rate of a fixed-orifice cryocooler is determined by the pressure in the vessel. Thus, the reduction of the pressure in the vessel during operation reduces the flow rate, the pressure in the evaporator varies, and the cooling temperature changes as well. In order to reduce the flow rate immediately after cool-down for depressing the cold temperature variations during the steady operation, a regulation system is required. In this paper we describe the development of a new flow controller, patent pending, for fast cool down cryocoolers that is a benefit of practical considerations. The new flow controller is designed for miniature cryocoolers, has high reliability, is maintenance friendly, and provides fast reduction of the flow rate after cool down

Analysis of ideal sorption compressor cycles operating with gas mixtures

Sorption-based compressors are thermally driven and because of the absence of moving parts they are vibration free, and have the potential for long life. Sorption-based compressors have been reported to operate Joule–Thomson (JT) cryogenic coolers with pure working fluids. However, using mixed refrigerants instead of pure refrigerants is attractive since that would dramatically improve the system coefficient of performance. Our on-going research aims to develop an efficient JT sorption cryocooler, operating with mixed refrigerants, and is focused on studying the characteristics of the sorption compressor cycle. This paper presents the results of an advanced numerical analysis, which is based on a previous model, and its experimental verification. The analysis relates to the ideal cycle of a sorption compressor operating with a gas mixture. Obviously, dynamics and kinetics play a major rule in a real sorption compressor cycle. However, since there are no reported gas-mixture sorption compressors and the existing experience in this field is poor, a preliminary ideal cycle analysis is considered. Satisfying agreement between the numerical and experimental results is obtained and the processes in the sorption cycle are discussed. The outcomes of the current study are the basis for the next phase in which a sorption compressor prototype will be built operating with gas mixtures

Modeling the adsorption of mixed gases based on pure gas adsorption properties

Sorption-based Joule-Thomson (JT) cryocoolers usually operate with pure gases. A sorption-based compressor has many benefits; however, it is limited by the pressure ratios it can provide. Using a mixed-refrigerant (MR) instead of a pure refrigerant in JT cryocoolers allows working at much lower pressure ratios. Therefore, it is attractive using MRs in sorption-based cryocoolers in order to reduce one of its main limitations. The adsorption of mixed gases is usually investigated under steady-state conditions, mainly for storage and separation processes. However, the process in a sorption compressor goes through various temperatures, pressures and adsorption concentrations; therefore, it differs from the common mixed gases adsorption applications. In order to simulate the sorption process in a compressor a numerical analysis for mixed gases is developed, based on pure gas adsorption characteristics. The pure gas adsorption properties have been measured for four gases (nitrogen, methane, ethane, and propane) with Norit-RB2 activated carbon. A single adsorption model is desired to describe the adsorption of all four gases. This model is further developed to a mixed-gas adsorption model. In future work more adsorbents will be tested using these four gases and the adsorption model will be verified against experimental results of mixed-gas adsorption measurements

Experimental verification of a numerical model for predicting the adsorption of mixed-gases

An ongoing effort is invested on developing a sorption compressor to drive Joule-Thomson cryocoolers with mixed-refrigerants. In previous work a numerical model is presented for determining the adsorption of mixed-gases based on the pure component adsorption characteristics. In the present paper an experimental verification of the numerical results is described. The experimental setup is suitable for simulating the sorption cycle in a compressor. The current research is focused on binary mixtures containing nitrogen and alkanes