Mesoscopic modeling and analysis of flow and heat transfer around a roughness-controlled string-grid wire

String-grid wires are typical fine-scale heat-transfer and flow-disturbance elements in wet string grids, air-handling systems, and gas–liquid heat and moisture exchange devices. Their surface structures directly affect near-wall boundary-layer development, wake-vortex evolution, and the overall heat-transfer performance. To reveal the effect of regular roughness on flow and heat transfer around a string-grid wire, a two-dimensional model with circumferentially arranged roughness elements was developed. A double-distribution-function D2Q9 lattice Boltzmann model was established to solve the flow and temperature fields, while the rough fluid–solid boundary was treated using solid-region mask identification and a halfway bounce-back scheme. The effects of relative roughness e/D on the velocity field, vorticity field, drag coefficient, temperature field, and average Nusselt number were systematically analyzed. The results show that roughness elements significantly modify the near-wall flow and wake-vortex structures by inducing local acceleration, flow separation, and enhanced wake disturbance. As e/D increases, the time-averaged drag coefficient rises from 1.57 for the smooth wire to 2.48 at e/D = 0.16, indicating a substantial increase in flow resistance. The average Nusselt number first decreases slightly and then increases with increasing roughness. At e/D = 0.12 and 0.16, the average Nusselt numbers reach 6.55 and 6.68, respectively, both higher than that of the smooth wire, 6.22. The simulated fields suggest competition between local stagnant shielding and thermal-boundary-layer renewal; this is a qualitative physical interpretation rather than a directly quantified mechanism. At e/D = 0.16, the Nusselt-number gain is approximately 7.4%, whereas the drag-coefficient increase is approximately 58.0%. The findings provide a numerical basis for evaluating roughness structures on string-grid wire surfaces and designing fine-scale enhanced heat-transfer elements.