Enhanced thermal energy storage using micropolar fluids: A numerical study

This study develops and solves a micropolar-fluid model for heat storage in a rectangular duct subject to constant wall heat flux. The dimensionless mass, linear and angular momentum, and energy equations are treated with a Runge-Kutta (for the coupled momentum ODEs) and finite-difference scheme (for the energy PDE). Increasing the coupling parameter raises the dimensionless axial velocity while initially reducing and then increasing microrotation; the net effect is a decrease in dimensionless temperature, indicating diminished storage effectiveness. In contrast, higher spin-gradient viscosity reverses these trends and enhances thermal storage. Reynolds and Prandtl numbers exhibit inverse relationships with the temperature profile. Quantitatively, the mean Nusselt number is <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mo>≈</mml:mo> <mml:mn>2.53</mml:mn> </mml:math> at an entrance length of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">x</mml:mi> </mml:mrow> <mml:mo>*</mml:mo> </mml:msup> <mml:mo>≈</mml:mo> <mml:mn>5.5</mml:mn> </mml:math> , while the local Nusselt number decays approximately exponentially downstream and approaches zero. For a low-inertia case ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">Re</mml:mi> </mml:mrow> <mml:mo>=</mml:mo> <mml:mn>5</mml:mn> </mml:math> ), the peak fluid temperature reached <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mo>∼</mml:mo> <mml:mn>95</mml:mn> <mml:mo>∘</mml:mo> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> </mml:mrow> </mml:math> under the imposed heat flux. These results clarify how micropolar parameters govern heat transfer and storage performance, offering guidance for tuning ducts that use micropolar working fluids.