Nanofluids it is a relatively new category of thermal fluids that can serve as coolants, both in single- and two phase systems. However application of nanofluids in industrial applications requires faultless their thermophysical properties. Existing models originally developed for liquid/solid particles mixtures (slurries) fail in case of nanofluids. Moreover correlations used to calcuate heat transfer coefficients of conventional fluids can not be used in case of nanofluids as well. Therefore comprehensive investigations of thermophysical properties of nanofluids as well as studies concerning heat transfer of nanofluids are still indispensable. In this paper reults of the simultaneous measurements of essential thermophysical properties like dynamic viscosity and thermal conductivity of water based nanofluids are presented. The results of the experimental investigations of the single phase forced convection inside horizontal tubes and pool boiling characteristics are presented as well. Finally the experimental results of the performance of the prototype two-phase thermosyphon heat exchanger (TPTHEx) and selected type of brazed plate heat exchanger (PHEx) operated with nanofluids are reported.
In the present paper, a detailed numerical investigation has been carried out to analyze the flow maldistribution in 50 parallel 1 mm x 1 mm rectangular minichannels and 1 mm depth minigap section with rectangular, trapezoidal, triangular or concave mani-folds in Z-type flow configuration (16 different cases). The working medium was etha-nol and the mass flow rate was 5×10-4 kg/s. Both sections was heated from the bottom side. Heat flux of 10 000 W/m2 and 5 000 W/m2 was applied to the minichannel and minigap section respectively. It provides an equal heat flow rate of about 50 W for both types of the sections. A novel approach for the mitigation of non-uniformity has been proposed introducing threshold at the entrance of the minigeometry section. The mal-distribution coefficient can be reduced about twice in the minigap section or about three times in the minichannel section with the 0.5 mm threshold as compared to the conven-tional arrangement without threshold. Authors analyzed velocity profile and tempera-ture profile over the heat exchanger’s surface. Reduction of maldistribution results in lower maximum temperature over the surface. The distribution is more uniform in min-ichannel section than in minigap section. This is due to a two-dimensional flow over a minigap in comparison to one-dimensional flow in channels. To obtain uniform distri-bution of fluid and hence uniform temperature profile, flow should be stabilized al-ready at the inlet manifold, at the entrance to the minichannel or minigap section.
The paper describes a fuel cell based system and its performance. The system is based on two fuel cell units, DC/DC converter, DC/AC inverter, microprocessor control unit, load unit, bottled hydrogen supply system and a set of measurement instruments. In the study presented in the paper a dynamic response of the proton exchange membrane (PEM) fuel cell system to unit step change load as well as to periodical load changing cycles in the form of semi-sinusoidal and trapezoidal signals was investigated. The load was provided with the aid of an in-house-developed electronic load unit, which was fully PC controlled. The apparatus was commissioned by testing the steady-state operation of the module. The obtained eﬃciency of the fuel cell shows that the test apparatus used in the study provides data in substantial agreement with the manufacturer’s data.
In this paper, forced convection of water/EG-Al2O3 nanofluids inside horizontal stainless steel tube is studied experimentally. As base fluid served water/EG mixture of two ratios (60:50 and 50:50). Nanoparticle mass concentrations was 0.1% or and 1%. Transition and turbulent flow regimes were tested. Average heat transfer coefficient and pressure drop values were determined for nanofluids and compared to base fluids. Deterioration of heat transfer for all tested nanofluids has been recorded compared to base fluid. Negligible increase of pressure drop for nanofluids compared to base fluids has been observed.
This paper presents thermal characteristics of prototype of a two-phase thermosyphon heat exchanger (TPTHEx) charged with acetone as a working fluid. The TPTHEx consists of two horizontal cylindrical vessels connected by two risers and a downcomer. Tube bundles placed in the lower and upper cylinders work as an evaporator and a condenser, respectively. The tested TPTHEx operates in a vacuum. Therefore, the working liquid is boiled in temperatures ranging from 33ºC to 62ºC. The overall heat transfer coefficient (OHTC) of the tested TPTHEx was estimated by the use of the Wilson method and the modified Peclet equation. The results obtained indicate a superiority of water over acetone as a working fluid. Moreover, it was shown that having a lower pressure in the shell-side of TPTHEx results in a higher overall heat transfer coefficient. The Wilson method and the modified Peclet equation predict OHTC with satisfactory agreement.
This work presents an experimental study of thermal energy storage by use of PCM. The aim of the study was to establish the influence of different inlet temperature of heat transfer fluid (HTF) and different Reynolds number of HTF on intensity of charging process. The PCM used in this study was stearic acid and water was used as HTF. A copper helical coil mounted in cylindrical container served as a heat transfer surface.
Politechnika Gdańska realizuje projekt Zintegrowany Program Rozwoju Politechniki Gdańskiej (POWER 3.5) dofinansowywany z Europejskiego Funduszu Społecznego. Celem projektu jest podniesienie jakości kształcenia na studiach II i III stopnia, zwiększenie efektywności zarządzania Politechniką Gdańską oraz podniesienie kompetencji kadr. Projekt jest realizowany od 1 kwietnia 2018 roku do 31 marca 2022 roku. Instytucją pośredniczącą w finansowaniu jest Narodowe Centrum Badań i Rozwoju.
The characteristics of alternative fuel for diesel: dimethyl ether (DME) and an analysis of the potential to reduce emissions of toxic compounds by the engines when running on this fuel have been presented in this work. Basic types of design solutions of gas supply and control systems, possible for use in internal combustion engines with DME supply have been also presented. The paper presents a study of the legislation and an analysis of the feasibility of the system storage and fueling DME on board small vessels.
In this work, Nusselt number and friction factor are calculated numerically for turbulent pipe flow (Reynolds number between 6000 and 12000) with constant heat flux boundary condition using nanofluids. The nanofluid is modelled with the single-phase approach and the simulation results are compared with experimental data. Ethylene glycol and water, 60:40 EG/W mass ratio, as base fluid and SiO2 nanoparticles are used as nanofluid with particle volume concentrations ranging from 0% to 10%. Nusselt number predictions for the nanofluid are in agreement with experimental results and a conventional single-phase correlation. The mean deviation is in the range of 5%. Friction factor values show a mean deviation of 1.5% to a conventional single-phase correlation, however, they differ considerably from the nanofluid experimental data. The results indicate that the nanofluid requires more pumping power than the base fluid for high particle concentrations and Reynolds numbers on the basis of equal heat transfer rate.
The paper describes a fuel cell based system performance under different thermal conditions. The system could be fed with bottled hydrogen or with very high purity hydrogen obtained from reforming of methanol. The system is based on two fuel cell units (1.2 kW each, produced by Ballard Power Systems Inc. and called Nexa), DC/DC converter, DC/AC inverter, microprocessor control unit, load unit, bottled hydrogen supply system and a set of measurement instruments. In this study steady-state operation of the PEM fuel cell system at different values of air excess ratio and different stack temperature was investigated. The load of the system was provided with the aid of a set of resistors. The results obtained show that the net power of the system does not depend on the air excess ratio within the range of from1.9 to 5.0. The polarizationcurves of the fuel cell module showed that the fuel cell performance was improved with increased stack temperature within the range of 30◦C to 65◦C. It was established that the total efﬁciency of the tested system depends on the hydrogen source and is higher when using bottled hydrogen of about 30% and 16%, for minimum and maximum load, respectively.