Performance of heat pipes as capillary pumps: experiments

Performance of heat pipes as capillary pumps: experiments Abstract This paper describes the development of an experimental setup to investigate the performance of heat pipes as capillary pumps. The experimental setup and the research that was performed by LFME was part of an EU funded research project called TRI-GEN EGD that aims in the overall development of a novel Tri-generation Electrogasdynamic converter system. The capillary pump will be used to pump the working fluid of the system using external waste heat. Furthermore, since the capillary pump has no moving parts it will provide the system with greater reliability. The experimental setup designed and constructed by LFME is a Capillary Pumped Loop (CPL). A CPL is a closed loop system that pumps liquid by passive means, based on the operating principles of heat pipes. Specifically capillary forces are generated on a porous structure that exists in the evaporation section and is responsible for pushing the working fluid from a high temperature source to a low temperature sink. In this paper, a brief description of the experimental setup that was constructed and a description of the experimental procedure that was followed will be given together with some results that were obtained for various configurations. Keywords capillary pumps; CPL; electrogasdynamic converter system 1. Introduction The work described in this paper was carried out as part of the EU funded research project, TRI-GEN EGD, which aims at the development of a novel Tri-generation Electrogasdynamic converter (TRI-GEN EGD) system and completes the work presented in a previous publication [1]. The TRI-GEN EGD system could be used to provide electricity, cooling and heating for buildings and industrial applications. It is based on the integration of an ejector refrigeration cycle with an electrogasdynamic (EGD) energy converter which is used to convert thermal energy into electrical power. The system could employ ‘ozone-friendly’ refrigerants such as water, methanol, ethanol, hydrocarbons or hydrofluoroethers (HFEs). In addition, the system has no moving parts and both ejector and EGD converter would be simple and reliable. The anticipated cost of production is low since inexpensive construction materials (e.g. copper or aluminum) could be used. The capillary pump of the TRI-GEN EGD system, investigated by the Laboratory of Fluid Mechanics and Energy (LFME) and presented in this paper, could be used to pump the working fluid of the system using external waste heat. The operation of the capillary pump is based on a Capillary Pumped Loop (CPL) system. International Journal of Low Carbon Technologies 2/1 Zisis G. Diamantis1, Dionysios I. Photeinos1, Professor Dionissios P. Margaris2, Professor Demos T. Tsahalis1 1 Laboratory of Fluid Mechanics and Energy (LFME), Department of Chemical Engineering, University of Patras, P.O. BOX 1400, 26500, Patras, Greece E-mail: , web page: http://lfme.chemeng.upatras.gr 2 Laboratory of Fluid Mechanics, Department of Mechanical Engineering and Aeronautics, University of Patras, 26500, Patras, Greece E-mail: , web page: http://fml.mech.upatras.gr Performance of heat pipes as capillary pumps: experiments 31 To this end, the present paper is structured as follows: initially a general description of the TRI-GEN EGD system is given and a brief introduction to CPLs is given in order to familiarize the reader with the technology being used. Next, a detailed description of the experimental setup is given together with a description of the instruments that are used. The experimental procedure will then be described and finally some results will be presented. 2. General description of the TRI-GEN EGD system and background theory on CPLs International Journal of Low Carbon Technologies 2/1 2.1 General description of the TRI-GEN EGD As mentioned, the TRI-GEN EGD (Fig. 1a) system could be used to provide electricity, cooling and heating for buildings and industrial applications as indicated in [2] and [3]. The proposed system consists of an EGD ejector unit (single nozzle or multi nozzle), a capillary pump generator (which is considered as the primary vapour generator using an external heat source, such as waste heat for example), a condenser and a secondary evaporator used to create the cooling effect. The capillary pump generator uses the outer loop and the secondary evaporator uses the inner loop as shown in Figure 1a. The proposed system has no moving parts thus making it more robust and at the same time minimising the creation of noise and vibration. The EGD converter part for creating electricity (Fig. 1b) consists of a corona electrode, a Venturi nozzle combined with an attractor electrode, and a condenser integrated with a collector electrode and works in the following way: initially the working fluid is vaporised in the capillary pump generator and the vapour flows to the Venturi nozzle, expanding and partially condensing. As the under-saturated vapour passes through the space between the corona and the attractor electrodes at a high speed, the created liquid droplets are charged due to the high electrostatic field and form a charged aerosol. As this high speed charged aerosol flows through the electrical field existing between the two electrodes, the kinetic energy is converted to electrical power. After discharge, the neutral aerosol condenses by cooling and the liquid working fluid is in turn pumped back to the vapour generator. In order for the system to incorporate cooling in addition to electricity production, the EGD converter part also contains an injection nozzle, a mixing section and a diffuser. After passing through the nozzle, the high pressure vapour expands to a high velocity and low pressure stream which combines with low-pressure vapours from the secondary evaporator causing a cooling effect. Both streams mix and diffuse to a higher pressure and condense in the condenser, which is placed right after the diffuser. From the above described procedure it can be clearly seen that the system can be used to produce electricity and provide heating and cooling simultaneously. Furthermore, as indicated above, there are no moving parts in the described system. Thus, with the introduction of a capillary circulation pump, the system would be completely passive. Such a pump would also reduce the number of components for the whole system because at the same time the working fluid would be pumped and vaporised, thus there would be no need for an additional generator in 32 Zisis G. Diamantis et al. High voltage source Heat out (heating) Load Condenser Heat in (cooling) Heat input, such as waste heat Ejector Evaporator Figure 1a. Expansion valve Schematic diagram of the TRI-GEN EGD system, indicating the most important points. Corona electrode Attractor electrode Charged aerosol Vapour Neutral flow High voltage source Load Figure 1b. Collector electrode Principle of EGD converter. the syst (...truncated)