CFD Solves Forging-Cooling Problem
Thermal-processes involve the effects of complex factors such as fluid flow, heat transfer, combustion, temperature control, particulate control etc. In many instances, the attempt to optimize a process requires time-consuming trial-and-error methods to evaluate different process parameters. By comparison, computation fluid dynamics (CFD), a numerical modeling technique, offers a powerful tool to take into consideration a large number of variables in simulating the process to arrive at an optimum process design in a much shorter time.
Airflow Sciences Corp. (ASC) has used CFD, together with its in-house wind tunnel, laboratory research and field testing to design cost-effective solutions to a wide variety of thermal processing-related problems, such as BOP exhaust systems, electrostatic precipitators, ductwork, fans, combustion systems, radiant heating systems, carburizing furnaces, annealing ovens and cooling enclosures.
To illustrate how CFD can assist in the evaluation of many design variables, ASC used CFD to assist in the thermal design of an enclosure to cool crankshafts and to eliminate the need for post-forging heat treatment. Ford Motor Co. wanted to eliminate post-forging heat treatment for crankshafts at one of its plants, and also wanted to cool the crankshafts on the existing conveyer line. The numerical technique allowed ASC to experiment with different design concepts, and the simulation predicted the cool down rates of the crankshafts as they passed through the simulated enclosure. In addition, the numerical technique helped ASC modify the existing concepts based on the detailed simulation results. A number of designs, weather conditions and plant operating conditions were evaluated to ensure that the final design worked for a wide range of conditions.
One approach to eliminating post-forging heat-treating is to control the cool-down rate in a manner to achieve the desired material mechanical properties, such as by passing the forged parts through a cooling enclosure. The enclosure not only controls the cool-down rate of the crankshafts, but also removes the thermal heat load of the cooling crankshafts from the plant. The use of numerical modeling to design the crankshaft cool-down enclosure involves generating a computer model, which is then used to evaluate design concepts before constructing the enclosure.
In the case discussed here, the conveyor line was located between the forge presses and the finishing equipment in an unenclosed area inside the plant. Forged crankshafts were loaded onto carriers suspended from the conveyor line so they could cool down to a temperature suitable for handling by factory workers.
The approach adopted for modeling the enclosure involved two basic steps. The first was to generate a model of a single crankshaft that accurately predicted the time-temperature history of a cooling crankshaft. The second was to use the single crankshaft model in conjunction with a cooling enclosure model, loaded with crankshafts, to evaluate the performance of enclosure designs. The crankshaft cooling-rate goals are those of a crankshaft cooled in still ambient air through transition, because empirical data collected for this cooling rate indicated that these are the conditions that produce the desired metal hardness and product quality.
During production, two crankshafts are placed on a carrier and suspended from the cooling conveyer. The objective of the single crankshaft model is to simulate one of the two crankshafts cooling on the carrier during still air cooling. A three-dimensional (3-D) CFD model was constructed of a single 6.8-liter crankshaft including a portion of the carrier. The model was then used to simulate the cooling of the crankshaft from the forging temperature to near ambient conditions.
Figures 1 and 2 show CFD predictions at two different times during the cooling simulation. These color contour plots are typical for reviewing CFD simulations results. Note that this is one "slice" out of the full three-dimensional model. The high temperature of the crankshaft immediately after forging is indicated by red in Fig. 1. The model predictions for the same slice after 30 minutes of still air cooling are shown in Fig. 2. This slice shows a notable decrease in the crankshaft temperature.
The CFD model not only predicts the internal temperatures of the crankshaft, but also the local air velocity around the crankshaft. The arrows in these figures are used to represent the local air velocity magnitude and direction. Note that the velocity is driven by thermal buoyancy such that when the crankshaft is hotter, the air velocities are higher (arrows are longer).
The time-temperature history of specific locations within an actual crankshaft was recorded during still air cooling. This empirical data were then compared with model predictions. Figure 3 details this comparison, indicating good correlation between the model and the experimental data. One additional point worth noting about these curves is that the model includes the phase-transformation effects of the steel as it cools (note the kink in the cooling curve at about 650 C, or 1200 F).
Cooling enclosure model
The exterior geometry of the cooling enclosure was dictated mainly by the physical constraints of the existing conveyor line and factory. Several design iterations were evaluated using the CFD modeling technique. Most of the designs targeted controlling the pretransition cooling of the crankshafts.
In general, the cooling enclosure is a rectangular box with two inlet ducts and one exit duct. Figure 4 shows a schematic of the final enclosure design. This schematic includes the path of the conveyor line in black. The crankshafts enter the enclosure through a door at the back of the enclosure from this view. They wind through two levels in the enclosure and exit the front of the enclosure. The air inlet ducts to the enclosure are shown in blue and the exit at the top of the enclosure is shown in red.
Note that the design uses no fans to move the air through the enclosure. Rather, the natural buoyancy of air is used. Natural buoyancy works on the concept that the forged crankshafts will heat the air in the enclosure causing it to rise and exit the enclosure through the vent at the top of the enclosure. The replacement air for the enclosure is drawn from the roof of the plant into the enclosure through the inlet ducts. Make-up air is, in turn, heated by the hot crankshafts, sustaining a steady flow through the enclosure and driven solely by natural buoyancy.
A full three-dimensional CFD model representing the entire volume of the cooling enclosure was constructed. A simplification of the detailed crankshaft/conveyer model was included in the enclosure model. Plant operation was assumed to be normal, so the enclosure was simulated fully loaded with crankshafts. The model results detail the temperature and velocity of the air in the enclosure, as well as the temperature history of the crankshafts as they pass through the enclosure.
A number of design concepts were considered using the CFD model. The primary goal of these designs was to control the cooling rate of the crankshaft during transition; in particular, to obtain the still air-cooling rate during transition (for product quality) and to accelerate the cooling rate after the transition region (for part handling).
Simulation results for the final design are detailed in Fig. 5. The colors shown in this cutting plane detail the air temperature as it is drawn into the enclosure passes, over the crankshaft, and out of the enclosure. Arrows also are included for presenting the model predictions of the air velocity within the enclosure. The important features of the final design were a partition wall and honeycomb flow straighteners, shown in gold at the rear top of the enclosure of Fig. 4. These items work together to protect the crankshafts during transition from the high velocity and turbulence of the bulk air flow passing through the enclosure.
The final design results for the bulk crankshaft temperature are shown in Fig. 6., which also shows the physical test data recorded for a number of points on and within the crankshaft during still air cooling. Comparison of the simulation results and the still air test data indicates that the design goals were achieved. The cooling rate before transition is similar to the cooling rates of still air indicating that desired material properties are obtained. In addition, the cooling rate after transition is faster than still air resulting in a cooler final crankshaft temperature for worker handling.
One additional curve on this figure (in green) details the results of the first design model considered during the design process, in which no flow control devices where included. Note that the cooling rate during the transition is much faster than the still air-cooling data for this design, indicating that the design goals of the enclosure would not be met with this design and that further design efforts would be necessary.
The enclosure worked as predicted by the model and did not require any additional modifications after construction to achieve the design goals. The desired product quality and hardness was achieved, eliminating the necessity for a post-forging heat treatment.