動樑式鋼胚加熱爐之熱流場模擬分析

Abstract

本文利用STAR-CD軟體，對鋼胚在動樑式加熱爐內之加熱過程建立三維穩態模擬工具。數學模式採用高雷諾數Favre平均之κ-ε紊流模式以及單一擴散係數、瞬間反應之PPDF紊流燃燒模式；熱輻射傳輸方程式則以離散座標法來計算。氣體之輻射吸收係數乃利用WSGGM (weighted-sum-of-gray-gases model)求得。模擬考慮了完整的加熱爐元件及外型，包括動靜樑系統、鋼胚、燃燒器、下擋牆、下煙道等。本文在第一部份以量測的鋼胚表面溫度做為邊界條件求解加熱爐內流場及溫度場，第二部份將鋼胚以高黏滯係數之層流流體模擬，假設鋼胚為連續流動，以耦合計算，同時求得加熱爐內燃氣流場、溫度場及鋼胚溫度分佈，第三部份則針對靜樑位置及墊塊(skid button)高度做參數分析，以求得最好的加熱爐結構，減少冷痕。在計算中動樑固定不動，因此可以穩態假設求解。 計算結果，加熱效率與量測值誤差在6%以下，而六個溫度監控點計算值與量測值，除下加熱區外，誤差均在10% 內。以量測的鋼胚表面溫度為邊界條件，可成功計算出加熱爐的流場，但量測值會影響計算結果。第二部份執行耦合計算時，可同時得到鋼胚表面溫度，除上表面外均十分吻合。 研究結果顯示鋼胚冷痕主要由動靜樑輻射遮蔽作用所產生，而墊塊將熱傳向冷卻水的作用則更加深冷痕的程度。另一方面，參數研究結果顯示，若在均溫區將靜樑向內偏置(減少兩靜樑間距)會增加鋼胚下表面的輻射遮蔽作用，冷痕因而變得更嚴重；反之，若將靜樑向外側偏置(增加兩靜樑間距)，則可以提高原本因輻射遮蔽作用加熱不佳區域之加熱率，冷痕因而改善。此外，研究結果亦顯示將墊塊的高度增加，除可減少動靜樑對鋼胚的輻射遮蔽作用，也可以增加墊塊本身的輻射熱傳量，墊塊溫度因而提昇，熱傳導量因而降低，冷痕因此獲得改善。

In the present study, a three-dimensional simulation is performed for the turbulent reactive flow and radiactive heat transfer in the walking-beam type slab reheating furnace by STAR-CD software. The study employs the high-Reynolds-number k-ε turbulence model based on Favre-averaged governing equations. The pre-assumed PDF model associated with the fast chemistry assumption and a single diffusivity is used to account for turbulent combustion. The absorption coefficient of the gases mixture is calculated by WSGGM (weighted-sum-of-gray-gases model). The discrete ordinates radiation model is adopted to calculate the radiactive heat transfer. The geometric model takes care of all components of the furnace, including the burners, the walking beam system with skid buttons, the slab, the dam, the down-take, etc. The part 1 of the study, the surface temperature of the slab is prescribed via experimental measurements and the furnace wall is assumed adiabatic. The turbulent reactive flow is thus simulated. The part 2 of the study, the temperature distributions of the slab and the gas mixture are obtained through a coupled calculation. The slab is modeled as a laminar flow having a very high viscosity and thus moving at a nearly constant speed. No radiation is concerned within the slab. The temperature distributions of the slab and the gas mixture are obtained through a coupled calculation. And the part 3 of the study, a solution for improving the skid mark by varying either the distance between two static beams or the height of the skid buttons is targeted. To obtain a steady solution, the walking beams are assumed fixed in the furnace. The simulation results agree with the measurements very well. The difference between the predicted heating efficiency and the measured one of the furnace is only 6%. The prediction errors at six temperature-monitored points are all under 10%, except the one in the lower heating zone which appears to be 13%. The measured surface temperatures of the slab were used as boundary conditions and the flow field of the reheating furnace can be obtained. However, the measured temperature will affect the result of calculations. When coupled calculations are executed in the part 2 of the study, the surface temperature can be obtained simultaneously. Except for the upper surface temperature, all the other simulation results agree with the measurements very well. Most of all, the influence of the walking beam system on the skid marks is thoroughly explored. The simulation results show that the radiative shielding by the static beams is the main cause of the skid mark. The heat loss through the skid button to the cooling system worsens the skid mark. A parametric study then shows by shifting the static beams inward in the soaking zone, the skid mark gets even worse due to an enhanced radiative shielding on the lower surface of the slab in between the two static beams; the skid mark on the opposite can be improved by shifting the static beams outward in the soaking zone. Another parametric study shows the skid mark can also be improved by increasing the height of the skid button.