熱輻射實驗特性分析及模式建立－中小尺寸火源

火焰之輻射熱在火災安全上扮演著相當重要的角色，在過去數十年裡已有不少研究投入於建立熱輻射的模式與實驗。然而，大部份之熱輻射模式並不適用於中、小尺寸之火災，亦即火源之有效直徑小於1 m；且在火焰輻射熱的數值模擬相關研究方面，文獻較缺乏且準確性方面仍有待提升。本研究藉由實驗與數值模擬對於火焰之輻射熱作一驗證，並根據不同之熱源大小(D = 14-38 cm)及熱輻射量測位置進行分析。在數值模擬方面，根據不同的格點尺寸、光譜波段(spectral bands)與固體角(solid angles)等參數，來尋求在進行數值模擬時之最適化操作條件。在火焰高度方面，經實驗與數值模擬結果之比對，當D*/δx小於13時其火焰高度是隨著縮小格點尺寸而增加；當D*/δx值超過13時，其火焰高度已呈現定值的趨勢。在實驗與數值模擬結果的比對方面，當量測點的高度低於火焰平均高度時，其結果的趨勢有很好的一致性；然而，當量測點的高度超過火焰平均高度時，計算的預測值多已高估了實驗值。其原因在於因燃燒模式的限制，使得火焰面周圍產生的煙量遠多於經實驗觀察的現象，過多的煙量可能導致其有高估的現象。在間歇區火焰之熱輻射方面，經由實驗量測獲得數據後可以發現，當輻射熱量測點置於燃料面的高度時，間歇區火焰所放射之輻射熱約為整體輻射熱值之36%；當量測點升高至火焰平均高度時，間歇區火焰所放射之輻射熱將約為整體輻射熱值之50%。此外，根據實驗結果所建立之熱輻射模式，結果顯示，針對本研究之實驗能獲得不錯之預測值；同時對於熱輻射熱分率之預測結果與實驗值比對，其誤差值約在14%的範圍內。本研究之最終目的在於修正數值模擬時之不準確度以尋求最佳操作條件，並建立適用於中小尺寸火源之輻射熱經驗式，期望應用於不同火場或初期火災時，能對輻射熱能有更高精確的掌握。

關鍵字

油池火災；火焰高度；輻射熱通量；間歇區火焰；計算流體力學

並列摘要

Radiant power from a flame plays an important role for fire safety. Over the last decades, numerous investigations of fires on thermal radiation have been developed by various thermal radiation models, numerical simulations and measurements. However, most of thermal radiation models are not suitable for small to medium sized fires, where the effective diameter of a fire is less than 1 meter. This work investigates the radiation characteristics in simulations and experiments from the heptane liquid pool fires. These small to medium sized pool fires (14-38 cm) were originated for comparison. The computational fluid dynamics (CFD) approach was used to predict the radiative heat flux for various pool diameters, grid sizes, spectral bands and solid angles. The results show that calculated flame shapes and heights are in good agreement with the measurements, as D*/δx is over 13. In the aspect of thermal radiation, the calculated radiative fluxes located within the height of the continuous flame zone are in good agreement with the measurements, whereas the predictions beyond that level deviate markedly from the experimental results, because the simulated amount of the soot created at the side of flame surface exceeds the experimentally observed volume. Experimentally, the effect of the intermittency of a flame on thermal radiation was performed. A shield was used to shade the radiant power from a persistent flame, and the radiative flux was measured from an intermittent flame on 30 cm diameter pool fires. The results show that 36 % of the radiative flux measured is emitted from the intermittent flame when the radiometer is located at the base of the pool fire. As the radiometer is moved upwards, the radiative fluxes measured from the intermittent flame increase gradually, even to 50%. Based on the results, the thermal radiation model with oscillation frequency was developed in this work. Measurements made with small to medium-sized pool fires (14-38 cm) were compared with estimates using the model. The results indicate that the predicted radiative fluxes closely correspond to the measurements made at various locations and that the variation of radiative fraction is within 14% as the location of the radiative flux gauge is varied.