The enhancement of axial heat and mass transfer by laminar flow oscillation in pipes with axial gradients in temperature and concentration has been studied analytically for the cases of insulated and conducting walls. The axial diffusivity can exceed its molecular counterpart by many orders of magnitude, with a quadratic scaling on the pressure-gradient amplitude and the Prandtl or Schmidt number, and is a bimodal function of oscillatory frequency: quasi-steady behavior at low frequencies and a power-law decay at high frequencies. When the pipe wall is conductive and of sufficient thickness, and the flow oscillation is quasi-steady, the axial diffusivity may be enhanced by a further factor of about ten as a result of increased radial diffusion, for liquid and gas flows in pipes with walls with a wide range of thermal conductivities. Criteria for the wall thickness required to achieve this additional enhancement and for the limits placed on the validity of these solutions by viscous dissipation are also deduced. When the heat transfer per unit flow work achieved by oscillatory pipe flow is contrasted with that of a conventional parallel-flow heat exchanger, it is found to be of comparable size and the ratio of the two is shown to be a function only of the pipe geometry, heat-exchanger mean velocity, and fluid viscosity.

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This work studies the thermal performance of a novel Spiral Solar Collector SSC experimentally with a mass flow rate ranged from 0.078 to 0.025 kg/s in an open system. A spiral-shaped tube of 13.5 m length, 0.01434 m diameter. The spiral shape is formed from six turns with an 11 cm gap distance between each spiral turn that is attached to an absorber plate. A new black paint mixed with a red sand powder is used to paint the mild steel absorber plate. The experiments are carried out in Al Ramadi city-Iraq under clear weather conditions during January and March 2022. Under different mass flow rates, two fluid flow cases are tested in the experiments; open system and close system cases. The proposed SSC represents a new alternative design that has a noticeable effect on thermal efficiency and outlet temperature. The results showed that there is an increase in the fluid outlet temperature with decreasing the mass flow rate. The difference between the experimental and theoretical results for the maximum value of the temperature difference is (17 K), which gave (6%) percentage error. It is found that the maximum temperature difference was 15 K in the open system case for a flow rate of 0.025 kg/s. According to the current findings, the thermal efficiency has increased up to 17% compared with other SSC and 27% related to conventional flat plate collectors.

The problem of fully developed laminar fluid flow in pipes, driven by an oscillatory pressure gradient, can be solved exactly for the time-dependent velocity field and related quantities such as flow rate and tidal displacement. When dissipation is neglected and the momentary axial variation of temperature is assumed to be linear, the corresponding thermal energy equation describing heat transfer along a pipe connecting two reservoirs at different temperatures can also be solved to yield exact solutions for the time-dependent temperature field, axial heat flux, and effective axial conductivity. In this paper, it is shown that these exact solutions for the unsteady temperature field are invalid at low Womersley numbers because the momentary axial variation of temperature is not linear. When the thermal energy equation is written in quasisteady form, approximate quasisteady analytical solutions can be found for the temperature field, which yield effective axial conductivities several orders of magnitude greater than those given by the low-Womersley-number, unsteady-flow solution. It is also shown that the conditions under which effects of dissipation on axial heat transfer become significant, at high Womersley numbers, can be determined by a simple criterion. When dissipation is significant, exact solutions for the unsteady temperature field are invalid at high Womersley numbers because the momentary axial variation of temperature is also nonlinear.