When designing a carbon fiber composite tubular graphite heat exchanger, controlling fluid resistance requires a comprehensive approach encompassing multiple dimensions, including fluid dynamics, tube-side structural optimization, shell-side flow path design, material surface treatment, improvement of local resistance components, control of operating parameters, and numerical simulation verification, to achieve a balance between efficient heat transfer and low-energy operation.
Flow resistance in a carbon fiber composite tubular graphite heat exchanger primarily stems from viscous friction and local obstructions. Viscous friction is directly related to fluid velocity, tube wall roughness, and flow path length, while local resistance is caused by structural changes such as elbows and inlet and outlet nozzles. During design, the fluid flow regime must be clearly defined, distinguishing between stagnant and turbulent flow regions. Friction coefficients can be controlled by adjusting flow path geometry. For example, in turbulent flow regions, tube wall roughness significantly increases the impact of resistance, making smooth carbon fiber composite tubes preferred to reduce friction losses.
Optimizing the tube-side structure is crucial for controlling straight tube resistance. The diameter, length, and arrangement of the carbon fiber composite tubes directly influence the fluid velocity distribution and pressure drop. A multi-channel, short-tube design shortens the fluid path and reduces flow resistance. Furthermore, optimizing the tube bundle density balances heat transfer area and flow channel unobstructedness, avoiding increased energy consumption due to increased fluid turbulence caused by overcrowding. Furthermore, a split-tube design can further reduce the flow velocity per tube pass, thereby minimizing straight-tube pressure drop.
Shell-side flow path design must balance heat transfer efficiency and flow resistance. For a shell-side configuration without baffles, when the fluid flows axially along the tube bundle, the resistance calculation can be simplified to a straight-tube resistance model, but additional consideration must be given to the local resistance of the inlet and outlet pipes and 90° bends. The introduction of baffles enhances shell-side flow turbulence and improves the heat transfer coefficient, but significantly increases flow resistance. Therefore, flow path analysis is required to accurately calculate the baffle spacing and segment height to find the optimal balance between heat transfer enhancement and increased flow resistance. For example, using spiral baffles instead of traditional arched baffles can reduce shell-side pressure drop and improve heat transfer uniformity.
Material surface treatment has a subtle impact on flow resistance. The finish of the inner wall of a carbon fiber composite tube directly affects the friction coefficient between the fluid and the tube wall. Reducing tube wall roughness through chemical polishing or coating can significantly reduce viscous frictional resistance. Furthermore, the interfacial bond strength between the graphite matrix and the carbon fibers also impacts the overall rigidity of the tube, preventing additional resistance caused by flow channel deformation due to vibration.
Improving local resistance components is key to reducing system pressure drop. The local resistance coefficient of components such as elbows and tees can be reduced by optimizing the curvature radius or implementing guide vanes. For example, replacing a standard 90° elbow with one with a larger curvature radius or adding guide vanes to guide the fluid smoothly can effectively reduce fluid separation and vortex formation, thereby reducing local pressure drop.
Operating parameter control requires a balance between heat transfer efficiency and fluid resistance. Increasing fluid flow rate can enhance turbulence and improve heat transfer coefficient, but it will significantly increase pipe straightness and local resistance. Therefore, the optimal flow rate range must be determined based on the fluid properties (such as viscosity and density) and process requirements. For example, for high-viscosity media, appropriately reducing the flow rate can reduce resistance while simultaneously compensating for decreased heat transfer efficiency by increasing the heat transfer area.
Numerical simulation technology provides a quantitative basis for controlling fluid resistance. By establishing a three-dimensional flow field model of a carbon fiber composite tubular graphite heat exchanger, it is possible to simulate fluid distribution, pressure drop, and heat transfer performance under varying structural parameters and operating conditions. Based on the simulation results, key design variables such as the number of tube passes and baffle layout can be optimized, avoiding the high cost and time-consuming traditional trial-and-error approach. For example, if simulations reveal that the shell-side pressure drop of a certain design exceeds expectations, this pressure drop can be reduced by adjusting the baffle spacing or using spiral baffles.
