Concluding remarks

The current study was covered several topics to help understand heat transfer and fluid mechanics in metal foams manufactured by the Replication method. The results of this study could help to develop high performance and low cost regenerators. The following list is a brief summary of the achievements and the work remains to be done.

11.1.1. Sample manufacturing:

1- Three different size salt particles (1-1.1 mm, 1.4-1.7 mm and 2.0-2.36 mm) were used to make aluminium foam with the Replication process.

2- The infiltrations method for replication process was investigated and two new methods, gas-only and mechanical infiltration, were introduced. It was found that samples manufactured by the gas-only method had better quality and process was more repeatable than the standard vacuum-gas and mechanical infiltration methods. 3- A total of eight samples (1.4-1.7 mm pore size) were manufactured under various

infiltration pressures to study the effect of infiltration pressure on samples pressure drop.

4- The optimum infiltration pressure for gas-only method was studied and the optimum infiltration pressure for three pore sizes, 1-1.1 mm, 1.4-1.7 mm and 2.0-2.36 mm, were identified.

5- A total of nine metal foams (three of each pore size) were manufactured by the gas- only method for thermal and pressure test. The manufactured samples had on average 63% porosity.

6- A packed bed of 10000 2mm ball bearing and a packed bed of 100 layers of wire mesh (No. 200) were built for comparing with metal foams samples.

11.1.2. Steady state pressure drop:

1- A test rig was built and the samples pressure drop was tested from 1 to 6.5 m/s. 2- The extended Darcy-Forchheimer equation was used to calculate the samples

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3- The results showed that samples’ permeability increased by increasing the pore size and the form drag reduced by increasing the pore size.

4- The Wire mesh sample had the highest normalised pressure drop following the Ball bearing sample.

5- The samples made under different infiltration pressure were tested and the results showed that the samples made under higher infiltration pressure than 4.5 bar had higher pressure drop and the samples made at lower infiltration pressure either partly infiltrated or had defects.

11.1.3. Oscillatory pressure drop:

1- A test rig was built and samples pressure drop were measured under oscillatory flow from frequency 1 to 19 Hz.

2- The results showed that the profile of the pressure drop for frequencies lower than 14 Hz were nearly sinusoidal due to the reciprocating motion of the piston but at higher frequencies the pressure drop profile became unsymmetrical notably where the piston accelerated (before 90o). Similar trends were observed for the second half of the cycle between 180o and 270o.

3- The maximum pressure drop occurred at 90o and 270o which was where the air velocity was at maximum. However, as the frequency increased the maximum pressure drop was shifted by a few degrees. This phase shift also increased by increasing the pressure drop.

4- Samples had higher pressure drop under oscillatory flow than steady state flow except for the Small samples which had higher pressure drop at steady state flow. 5- Darcy-Forchheimer equation was applied to the oscillatory pressure data and the

data showed that the reduced pressure drop only followed Darcy-Forchheimer regime for flow velocity higher than 0.5 m/s

6- Flow velocity was measured at the centre of the connecting pipe (between the compressor and the test section) by a hot-wire anemometer. The results showed that the air velocity fluctuated during the acceleration and deceleration period and behave “like a turbulent flow”.

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11.1.4. Heat transfer measurement:

1- A single-blow test rig was designed and manufactured to measure thermal performance of the samples. Samples were tested at Reynolds number from 250 to 1350. Reynolds number was measured based on foams pore diameter.

2- Several types of the single-blow model method (depending on working fluid properties and regenerator types) were presented and analysed. The extended Schumann-Hausen model was implemented for predicting the outlet air temperature.

3- The mathematical model was numerically solved by an explicit finite difference method and the results were further improved by an implicit method (The Crank- Nicolson) for better accuracy. The accuracy of the modelling results was validated by the Kohlmayer’s (Eq. 6.19) analytical solution.

4- The effect of changing NTU, NTUw, Rtc and inlet air response on the maximum

gradient was investigated and the results were plotted for visualising the effect of variables on NTU.

5- Four different matching techniques were evaluated and the maximum gradient method and direct curve matching were selected for match the experimental and predicted curves.

6- A new technique was introduced to match the experimental and predicted data with direct curve matching when the effective thermal conductivity, NTU and NTUw were

unknown.

7- Effective thermal conductivity of the foams was estimated for the first time with the direct matching technique and the results had good agreement with the data estimated based on sintered metal and high porosity metal foam correlation.

8- The metal foams results showed that NTU increased by decreasing the pore size and the results were similar for each pore size.

9- Samples heat transfer coefficients were calculated and plotted against Reynolds number. The results showed that based on mass flow rate Mid samples had the highest h, however the differences between the metal foams were insignificant.

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10- The Ball bearing and Wire mesh samples results were compared with Kays and London results and good agreement particularly with the Wire mesh results was achieved.