Augustin Lomena Mulenda, Vertomene Sumuna Temo, Bernard Ndaye Nkanka, Sadiki Amsini, Haddy Mbuyi Katshiatshia
Centre de Recherche en Energies Renouvelables, Faculté Polytechnique, Université de Kinshasa, Avenue de l'Université N° 01, Commune de Lemba, BP 127 Kinshasa, Democratic Republic of the Congo.
Centre d'Etudes et de Recherches sur les Energies Renouvelables Kitsisa-Khonde (CERERK), ISTA-Kinshasa, Avenue Aérodrome N° 3930, Commune de Barumbu, BP 6593 Kinshasa, Democratic Republic of the Congo.
Entropy (Basel). 2022 Jul 23;24(8):1019. doi: 10.3390/e24081019.
The chimney effect taking place in biomass cooking stoves results from a conversion process between thermal and mechanical energy. The efficiency of this conversion is assessed with the stove loss coefficient. The derivation of this quantity in cooking stove modelling is still uncertain. Following fluid mechanics, this loss coefficient refers to an overall pressure drop through stove geometry by performing an energy balance according to the first law of thermodynamics. From this approach, heat-transfer processes are quite ignored yet they are important sources of irreversibilities. The present work takes a fresh look at stove loss coefficient assessment relying on the second law of thermodynamics. The purpose in this paper is to identify the influence of operating firepower level on flow dynamics in biomass natural convection-driven cooking stoves. To achieve that, a simplified analytical model of the entropy-generation rate in the flow field is developed. To validate the model, experiments are conducted first on a woodburning stove without cooking pot to better isolate physical processes governing the intrinsic behaviour of the stove. Then, for the practical case of a stove operating with a cooking pot in place, data from published literature have served for validation. In particular, mass-flow rate and flue gas temperature at different firepower levels have been monitored. It turns out that losses due to viscous dissipations are negligible compared to the global process dissipation. Exergy analysis reveals that the loss coefficient should rather be regarded from now as the availability to generate flow work primarily associated with the heat-transfer Carnot factor. In addition, the energy flux applied as flow work has to be considered as pure exergy that is lost through consecutive energy-transfer components comprising the convective heat transfer to the cooking pot. Finally, this paper reports a satisfactory agreement that emerged between the exergy Carnot factor and the experimental loss coefficient at different fuel-burning rates.
生物质烹饪炉灶中发生的烟囱效应源于热能与机械能之间的转换过程。这种转换的效率通过炉灶损失系数来评估。在烹饪炉灶建模中，该系数的推导仍不明确。根据流体力学，通过依据热力学第一定律进行能量平衡，此损失系数指的是通过炉灶几何形状的总压降。从这种方法来看，传热过程完全被忽略了，然而它们却是不可逆性的重要来源。本研究基于热力学第二定律重新审视炉灶损失系数评估。本文的目的是确定运行火力水平对生物质自然对流驱动烹饪炉灶内流动动力学的影响。为实现这一目的，建立了流场中熵产生率的简化分析模型。为验证该模型，首先在没有烹饪锅的燃木炉灶上进行实验，以更好地分离控制炉灶固有行为的物理过程。然后，对于有烹饪锅的炉灶实际运行情况，已发表文献中的数据用于验证。特别地，监测了不同火力水平下的质量流率和烟气温度。结果表明，与整体过程耗散相比，粘性耗散造成的损失可忽略不计。火用分析表明，从现在起，损失系数更应被视为主要与传热卡诺因子相关的产生流动功的可用能。此外，作为流动功施加的能量通量必须被视为纯火用，它通过包括向烹饪锅的对流换热在内的连续能量传递组件而损失。最后，本文报告了在不同燃料燃烧率下，火用卡诺因子与实验损失系数之间出现的令人满意的一致性。
