Experimental and numerical study of the use of nanofluids in compact heat exchangers
Special Symposium - EPIC-1: European Process Intensification Conference - 1
EPIC-1: Intensified Hydrodynamics & Structured Environments (IHSE-1)
Keywords: plate heat exchanger, nanofluids, thermal conductivity, CFD
The need for efficient yet small in size heat transfer equipment has led to the development of compact heat exchangers with modulated surface, as well as to an increased research interest concerning the enhancement of the thermal capability of the working fluids. In this frame, the suspensions of millimeter- or micrometer-sized particles in conventional liquids have been examined in the past, with limited success for practical applications, due to inherent problems such as sedimentation, erosion, fouling and increased pressure drop in the flow channel. A new trend in the last decade is to enhance the thermal conductivity of the conventional working fluids by adding nanometer-sized solid particles. It seems that these suspensions, called nanofluids, exhibit a significant increase in the thermal conductivity of the base fluid, while they can probably overcome the aforementioned problems as the particles are ultra-fine and usually used at low particle concentrations (Trisaksri & Wongwises, 2007). Although much work has been done and published lately on nanofluids, it is mainly focused on the preparation methods and the thermal conductivity of the suspensions, rather than their heat transfer characteristics and flow behavior (Wang & Mujumdar, 2007). Experimental work in the convective heat transfer of nanofluids is still quite scarce, so further investigation is needed (Das et al., 2006).
In the present work, the performance of a nanofluid in a compact heat exchanger is studied experimentally and numerically and it is compared with that of conventional working fluids (e.g. water, ethylene glycol etc). The nanofluids are prepared using commercially available multi-wall carbon nanotubes, C-MWNT, or metal oxide nanoparticles, which are dispersed in water in the presence of a suitable surfactant and stabilized with ultrasonic vibration. Their physical properties are well defined, among which the thermal conductivity, measured using the transient hot-wire method, exhibits in the case of C-MWNT an increase of about 20% in comparison with pure water, for an 1% w/w suspension (Assael et al. 2004). A commercial liquid cooling system is employed and suitably adapted for the needs of the study. The main apparatus is a miniature compact plate heat exchanger, comprised of a small square copper plate, the one side of which has rods, is covered with a plastic case and creates the flow path for the cooling liquid. The other side of the copper plate is flat and is placed in contact with a heating source. The liquid flows with the help of a small pump and after passing through an air-cooled heat exchanger is recirculated. The temperature at various locations along the cooling system is simultaneously measured by several thermocouples and the data are collected by a PC through a terminal board with cold junction compensation and an A/D card.
In this Laboratory, previous experimental and numerical studies of a typical compact heat exchanger have proved that CFD is a reliable tool to predict flow characteristics and heat transfer rates, as well as pressure losses, in this type of process equipment (Kanaris et al., 2006). Thus, a commercial CFD code (i.e. ANSYS CFX® 10.0) is employed for the numerical evaluation of the performance of the aforementioned system with the use of nanofluids. The simulation is validated using experimental data on overall temperature differences, acquired for flow of nanofluids at the cold side of the model heat exchanger, for various heat duties. Information provided by this study, which is currently in progress, is expected to help the efforts to optimize the design of compact heat exchanging equipment working with nanofluids.
References
Assael, M. J., Chen, C.-F., Metaxa, I., Wakeham, W. A. 2004 Thermal conductivity of suspensions of carbon nanotubes in water. Int. J. Thermophysics, 25, 971-985.
Das, S., Choi, S., Patel, H. 2006 Heat Transfer in Nanofluids-A Review. Heat Transfer Engineering, 27(10), 3-19.
Kanaris, A.G., Mouza, A.A., Paras, S.V. 2006 Flow and heat transfer prediction in a corrugated plate heat exchanger using a CFD code. Chemical Engineering and Technology, 29 (8), 923-930.
Trisaksri, V., Wongwises, S. 2007 Critical review of heat transfer characteristics of nanofluids. Renewable and Sustainable Energy Reviews, 11(3), 512-523.
Wang, X.-Q., Mujumdar, A. S. 2007 Heat transfer characteristics of nanofluids: a review. International Journal of Thermal Sciences, 46(1), 1-19.
Presented Wednesday 19, 11:40 to 12:00, in session EPIC-1: Intensified Hydrodynamics & Structured Environments (IHSE-1).
