Although thermal convection is ubiquitous in nature and technology, it has long been argued whether convection can be considered as an independent mode of heat transfer. Here, we show that pure convection mode is generally recognized from the classical first and second laws via mass flow by rooting the unsteady mass conservation relation through an equivalent multiport system. The fundamental distinction is made between convection driven by the thermal and entropy potential differences and mass flow accompanied by zero potential difference. The constitutive relations of the convective heat and entropy fluxes and their thermodynamic relationship are developed in terms of their potential differences with the exponential distribution of temperature difference in a compressible flow. In the absence of work transfer, the first and second laws are simply expressed by the standard Maxwell’s electromagnetic type equations via the total heat and entropy flux vectors. The pure convection theory is applied to a parallel-flow heat exchanger and the linear temperature difference distribution is obtained for both streams via the first-law definitions of the convective and overall heat transfer coefficients in an internal flow. The entropy generation rate inside a heat exchanger is recalculated and indicated to be driven by the difference between the reciprocal of outlet temperatures of two streams, from which the entropy generation number between outlets is developed to eliminate the entropy generation paradox. The present linear temperature difference profiles agree well with the experimental data, and the outlet entropy generation number is also justified by comparing with the previous nondimensionalizing entropy generation numbers.

中文翻译：

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非定常多端口系统第一定律和第二定律基础的对流传热和熵传递基本机制及其在并流换热器中的应用

尽管热对流在自然界和技术中普遍存在，但长期以来人们一直在争论对流是否可以被视为一种独立的传热模式。在这里，我们通过等效多端口系统建立非稳态质量守恒定律，证明纯对流模式通常可以通过质量流从经典第一和第二定律中得到认可。由热势差和熵势差驱动的对流与伴随零势差的质量流之间存在根本区别。对流热通量和熵通量的本构关系及其热力学关系是根据可压缩流中温差指数分布的势差来发展的。在没有功传递的情况下，第一定律和第二定律可以通过总热和熵通量矢量简单地用标准麦克斯韦电磁型方程来表达。将纯对流理论应用于并流换热器，并通过内流中对流和总传热系数的第一定律定义获得两股流的线性温差分布。重新计算热交换器内的熵产生速率，并表明其由两股流出口温度倒数之间的差异驱动，由此得出出口之间的熵产生数，以消除熵产生悖论。目前的线性温差曲线与实验数据吻合良好，并且通过与之前的无量纲化熵产生数的比较，也证明了出口熵产生数的合理性。