Thermal analysis of a plastic helical coil heat exchanger for a domestic water storage tank

In the present study, the heat transfer coefficients of helically coiled corrugated plastic tube heat exchanger inside of the solar boiler vessel were investigated experimentally. The metal coil of the conventional solar boiler for domestic usage was replaced by a plastic tube and the results were compared with the numerical simulation and the technical documentation of the initial solar boiler. All the required parameters like inlet and outlet temperatures of tubeside and stratified temperatures, flow rate of fluids, etc. were measured using appropriate instruments. The test runs were performed for different temperatures inside the tank ranging from 30-60°C and different flow rates from which the heat transfer coefficients were calculated

Influence of pore density on thermal development in open-cell metal foam

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Experimental results for oscillatory water flow in 10-ppi metal foam at low-frequencies

This experimental study presents results and interpretation of oscillatory water flow in open-cell metal foam. The tested foam had 10 pores per inch and a porosity of 88%. At relatively low frequencies, three flow displacements were employed in the experiment. The influence of frequency and displacement on pressure loss and friction factor is discussed. A correlation of friction factor as a function of the kinetic Reynolds number was determined. Porous media parameters, permeability and drag coefficient, were also found for the same foam via steady-state flow experiments in the Darcy and Forchheimer regimes. The friction factor of oscillating flow was found to be higher than that of steady state. The findings of this study are considered important for oscillating heat transfer in metal foam

Parametric study of a triangular cross corrugated plate

The main goal of this study is to investigate the influence of apex angle and Reynolds number on the thermal hydraulic performance of triangular cross corrugated plates. More specifically this work focuses on triangular cross corrugated plate with the orientation angle of 90°. The computational fluid dynamics (CFD) method is used to conduct, three-dimensional simulations for 426 < Re < 2021 in a periodic unitary cell. The Reynolds Stress model is used as the turbulence model. The numerical results are in a very good agreement with experimental results correlation. They show deviations between 0.8 – 4.84 %. The highest thermal performances are achieved by the both apex angle of 120° and 90°. The lowest thermal performance is observed by the apex angle of 55°. The heat exchanger with the apex angle of 90° has the highest friction factor. For the Reynolds number lower than 1300, the apex angle of 120° shows the lowest friction factor. However for the Reynolds number higher than 1300, the apex angle of 55° shows the highest hydraulic performance.Papers presented at the 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Portoroz, Slovenia on 17-19 July 2017 .International centre for heat and mass transfer.American society of thermal and fluids engineers

Plastic helical coil heat exchanger as an alternative for a domestic water storage tank

A reduction in weight and cost of a domestic hot water storage vessels is an interesting case for the industry, that can be reached by an alternative material of the helical coil heat exchanger inside them. The goal of the present study was to design a fully polymer solar boiler demonstrator and to explore its thermal performance in the low pressure and low temperature conditions. The metal coil of the conventional solar boiler for domestic usage was replaced by a plastic tube and the heat transfer behavior of helically coiled smooth plastic tube heat exchanger was investigated experimentally. The heat exchanger is placed in the middle of the tank in two parallel coils that fill almost whole height of the vessel in order to achieve maximal surface area. The water inside the tank was heated by circulating in closed loop with heater to achieve constant initial temperature across the whole volume of the vessel. When reached, the heating was stopped and a cold water of the tap temperature started to flow inside the polymeric tube. All the required parameters like inlet and outlet temperatures of tube-side and stratified temperatures in fifteen different points, flow rate of fluids and pressure drop were measured using appropriate instruments. The test runs were performed for different initial temperatures inside the tank ranging from 30-60°C from which the overall heat transfer coefficient and thermal resistances were calculated. The validity of obtained results was compared with the numerical simulation and the experimental results on the initial metallic tube.Papers presented at the 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Portoroz, Slovenia on 17-19 July 2017 .International centre for heat and mass transfer.American society of thermal and fluids engineers

A discussion on the interpretation of the Darcy equation in case of open-cell metal foam based on numerical simulations

It is long known that for high-velocity fluid flow in porous media, the relation between the pressure drop and the superficial velocity is not linear. Indeed, the classical Darcy law for shear stress dominated flow needs to be extended with a quadratic term, resulting in the empirical Darcy–Forchheimer model. Another approach is to simulate the foam numerically through the volume averaging technique. This leads to a natural separation of the total drag force into the contribution of the shear forces and the contribution of the pressure forces. Both representations of the total drag lead to the same result. The physical correspondence between both approaches is investigated in this work. The contribution of the viscous and pressure forces on the total drag is investigated using direct numerical simulations. Special attention is paid to the dependency on the velocity of these forces. The separation of the drag into its constituent terms on experimental grounds and for the volume average approach is unified. It is shown that the common approach to identify the linear term with the viscous forces and the quadratic term with the pressure forces is not correct

Experimental hydrodynamics of high-porosity metal foam: Effect of pore density

Commercial open-cell metal foam has very high porosity (often greater than 90%) and a large surface area density. The open flow area is copious compared to the ligament size. These properties are exploited in many applications, e.g., heat exchanger, reactors and filters. Pressure drop, flow regimes, and transition from one to another, are indispensable for any application involving flow of a fluid through the foam, and for heat transfer rates or reaction paces. These topics are not well-agreed on for foam-like porous media such as metal, graphite and polymeric foams. Pressure drop parameters such as permeability and form/inertial drag coefficients are very divergent for metal foam; the same can be said about flow regime boundaries. This paper presents experimental data for pressure drop for water flow in two commercial open-cell aluminium foams having 10 and 40 pores per inch (ppi). The two foams have similar porosities (88.5%). The wide range of flow Reynolds number covered all known flow regimes in porous media: pre-Darcy, Darcy, Forchheimer and turbulent. Flow regimes and transition between them were identified and compared. The friction factor based on the square root of permeability (measured in the Darcy regime) and the Reynolds number based on the same characteristic length were used. It is shown that the same foam exhibits different values of its permeability and Forchheimer coefficient in different flow regimes. A previously-tested foam having 20 pores per inch and a porosity of 87.6% was included in the comparisons. The basic finding of this study will inform numerical and analytical work concerning flow and heat transfer in foam-like highly-porous porous media. (C) 2016 Elsevier Ltd. All rights reserved

Comparison of heat transfer and pressure drop between analytical and computational approaches: a preliminary study for optimal heat exchanger design

Extended surface areas are indispensable features for compact heat exchanger design. Even one of the simplest, plain-fin-and-tube heat exchangers are still widely studied and used due to their relatively-easy production compared to other types of fin geometries. However, this simplicity only means that there are fewer parameters to consider, compared to those louvered fins for example. Those parameters are the transversal and longitudinal pitches between tubes, fin pitch, fin thickness, inner and outer tube diameters and the number of tube rows, given whether an optimization scheme is required to find a design solution. In this study, the validity of analytical and 3D computational fluid dynamics solutions employing the aforementioned parameters was investigated as a preliminary step to optimal heat exchanger design. The causes of differences between analytical approaches and the associated experimental solutions from previous studies were also sought via simulations. For this purpose, geometric parameters extracted merely from the Reynolds numbers used in those studies, were used to construct a plain-fin-and-tube heat exchanger core. Care was taken so as to employ air velocities remaining in the laminar regime traveling between the fins. It was found that the bounds of the experimental parameters which had been used to define correlations, had a significant impact on the validity of the analytical approach. The three-dimensional model proved to generate viable results with respect to already-published experiments. Since this study constitutes the preliminary step for an optimization scheme, the findings are also accompanied by an extensive literature review on analytical and computational tools

Investigation of low-frequency-oscillating water flow in metal foam with 10 pores per inch

In this study, oscillating water flow in metal foam with open cells is investigated experimentally. The metal foam sample has a porosity of 88% and 10 pores. The water was oscillated in the test section with three frequencies between 0.116 Hz and 0.348 Hz, which are considered low for water oscillation, and three flow displacements ranging between 74.35 mm and 111.53 mm. The combinations of frequencies of displacements were studied for their impacts of dimensional and non-dimensional pressure loss quantities. To this purpose, friction factor was correlated as a function of kinetic Reynolds number. The same metal foam sample was studied by exposing it to steady-state water flow to investigate its permeability and drag coefficient in low-velocity flow regimes. The friction factor distribution for oscillating flow was found to be over that found for steady state. The outcomes of the study are important for studying heat transfer under the same flow conditions

Heat transfer measurements for non-Darcy flow in 10-ppi metal foam

Metal foam is a class of porous media with very high porosity (around 90%) and a large surface area density. The foam internal structure is web-like of thin ligaments surrounding cells that are open to flow. This structure promotes thermal dispersion because it offers a lot of mixing of a flowing fluid. Break up and inception of the boundary layers are phenomena adding to the convective heat transfer. The thermal conductivity of the solid phase of metal foam is also high. Because of all these attributed, metal foam is an excellent heat exchanger technology. There is a need for more experimental data regarding heat transfer in metal form. In this paper, experimental heat transfer data for water flow in commercial open-cell aluminum foam cylinder heated at the wall by a constant heat flux (14,998 W/m2 and 26,347 W/m2), is presented. The foam had 10 pores per inch (ppi) and a porosity around 87%. The measurements included wall temperature along flow direction as well as average inlet and outlet temperatures of the water. Flow speeds were in the non-Darcy regimes: transitional and Forchheimer flow regimes. The behavior of the wall temperature clearly shows thermal development conditions. The experimental Nusselt number is presented as a function of axial distance in flow direction, and showed what seemed to be periodic thermal development. The experimental data can be used for validation of other analytical solutions. The results can also be used to verify numerical models and metal-foam heat exchangers used in air-conditioning for example