Particulate fouling and challenges of metal foam heat exchangers

In recent years, open-cell metal foam has gained attention for utilization for exhaust gas recirculation coolers due to its large surface area and porous structure. Theoretically, the porous foam structure would have better transfer heat through conduction and convection processes. However, the exhaust gases that enter the cooler would carry particulate matter, which may deposit within the foam structure. The existing fouling studies cannot explain the underlying mechanisms of particulate deposition thoroughly within the foam structure. This study reviews the particulate fouling of heat exchangers, particularly in the exhaust gas recirculation system. Some past approaches to investigate fouling, particle transport, and deposition in the metal foam heat exchangers for many different applications are also included. In addition, this study also includes the challenges that lie ahead in implementing the metal foam heat exchangers in the industries

Temperature And Velocity Changes Across Tube Banks In One-Directional And Bi-Directional Flow Conditions

The back-and-forth movement of flow in oscillatory flow condition that can be found in blood flow, thermoacoustic energy system and ocean wave can be categorized as bi-directional flow condition and heat transfer in this flow condition is not well understood. This paper reports an experimental investigation that compares temperature and velocity values between the onedirectional (the usual flow condition) and the bi-directional flow conditions. The experiment was done using thermoacoustic’s standing wave rig with two different drivers to drive the one-directional flow and bi-directional flow conditionsin the test rig. Results, that were recorded using piezoresistive pressure sensor, type-K thermocouple and hotwire anemometer, indicate that care should be exercised when calculating heat transfer in bi-directional flow conditions as the temperature and velocity changes are different compared to the one-directional flow condition. Differences were recorded to be within the range of 77% for temperature and 59.5% for velocity, presumably due to the different behavior of forced and natural convection effect as flow conditions change

Flow visualization study of partially filled channel with aluminium foam block

This experimental study investigated fluid dynamics of a channel partially filled with a metal foam block with various pore densities; 5, 10 and 30 PPI. The effects of foam heights, in a range of 0.004 – 0.050 m, and inlet velocities, from 3.9 to 12.5 m/s, on the flow field were investigated using Particle Image Velocimetry (PIV). For a more comprehensive analysis of the problem, detailed flow patterns inside the porous foam were visualized by applying a thermal imaging method. Results show that there are different flow behaviours in the partially filled channel depending on the key parameters. A boundary layer is formed on the horizontal fluid/foam interface, which is much more noticeable and thicker in high blockage ratio cases. The low-pore density foam allows the flow to pass through its porous structure while the higher pore density foam imposes more restrictions on the flow pushing the fluid away from the foam into non-porous region on the top of the foam block before reaching the foam end

Experimental study of fluid flow behaviour and pressure drop in channels partially filled with metal foams

This study experimentally investigates the effects of pore density, inlet velocity and blockage ratio on fluid flow behaviour and pressure drop in channels partially filled with a metal foam block. The fluid velocities in the free stream region, which is a clear (from foam) region on the top of foam block, are measured using Laser Doppler Anemometry (LDA) and hot-wire anemometry. The metal foam data are compared to those of solid blocks with the same size. For low blockage ratios, i.e. thin foam layers, the pressure drop caused by a solid block is higher than that of the foam when tested under identical conditions. Interestingly, nonetheless, beyond a threshold blockage ratio value, the pressure drop induced by the metal foam block exceeds that of the solid block of the same height tested at the same air flow rate. This behaviour is best described as the interplay between resistance caused by blockage versus that of the wake forms downstream and over the objects and additional frictional effects within the porous region and on the interface. Furthermore, a correlation, with ±16% deviation, is developed to predict the flow resistance caused by the solid and foam blocks across the partially filled channel

Thermophysical Properties And Heat Transfer Performance Of Al2O3/R-134a Nanorefrigerants

The past decade has seen the rapid development of nanofluids science in many aspects. In recent years, refrigerant-based nanofluids have been introduced as nanorefrigerants due to their significant effects over heat transfer performance. This study investigates the thermophysical properties, pressure drop and heat transfer performance of Al2O3 nanoparticles suspended in 1, 1, 1, 2-tetrafluoroethane (R-134a). Suitable models from existing studies have been used to determine the thermal conductivity and viscosity of the nanorefrigerants for the nanoparticle concentrations of 1 to 5 vol.%. The pressure drop, pumping power and heat transfer coefficients of nanorefrigerant in a horizontal smooth tube have also been investigated with the same particle concentration at constant velocity of 5 m/s and uniform mass flux of 100 kg/m2 s. In this study, the thermal conductivity of Al2O3/R-134a nanorefrigerant increased with the augmentation of particle concentration and temperature however, decreased with particle size intensification. In addition, the results of viscosity, pressure drop, and heat transfer coefficients of the nanorefrigerant show a significant increment with the increase of volume fractions. Therefore, optimal particle volume fraction is important to be considered in producing nanorefrigerants that can enhance the performance of refrigeration systems

Numerical and experimental study of flow behaviours in porous structure of aluminium metal foam

This study conducted a simulation and an experimental study on a channel that was partly filled with open-cell metal foam block. Different blockage ratios have been considered, where the foam height could occupy more than half of the channel size. The aim is to investigate the flow behaviours across the complicated structure of the porous medium. Results show that the use of Ergun and Forchheimer models showed a similar stagnant region and recirculation zone in the partially filled channel, which agreed with the experimental results. However, the deviation in pressure drop performance at a high blockage ratio is noticeable

Uncertainty analysis of thermal fluid measurements for Bi-directional flow condition across tube banks

The uncertainty analysis for experimental investigation of bi-directional flow conditions of thermoacoustics is presented. The experimental rig used for loudspeaker as a flow inducer to provide acoustical flow across tube banks that is placed inside a standing wave resonator. The measured velocity and temperature changes within the vicinity of the tube banks are presented along with the uncertainty values. The standard deviation for velocity and temperature data shows that data varies with maximum deviation of 0.14 m/s and 5.78 °C, respectively. The results show that a good repeatability was obtained during the experiments which indicates that a reliable thermal-fluid measurement of bi-directional flow condition was achieved