Abstract
The use of micro-heat exchangers increased with the advancement of microfluidics. These microdevices present some advantages like elevated surface area-to-volume ratio resulting in high heat transfer rates. Micro-heat exchanger with phase change is a new application of such devices. The simultaneous momentum, heat, and mass transfer at microscale still require investigations due to the inherent complexity. The main goal of the chapter is to demonstrate experimentally and numerically the capability of the micro-heat exchanger use in the continuous process of ethanol excess recovery from the biodiesel. The influence of flow rate, ethanol/biodiesel molar ratio, and temperature on the ethanol evaporation performance was evaluated. The flow rate and the ethanol/biodiesel molar ratio influenced negatively the evaporation. In contrast, the temperature was affected positively. The mathematical model was able to capture the main features of the continuous evaporation; however, further improvements must be performed in order to consider the thermodynamics characteristics.
TopIntroduction
The use of microchannels in heat and mass transfer operations has received great attention in the last decades. The advantages of the microdevices regarding the conventional macroscale equipment, especially the high surface area-to-volume ratio, enhances the transport phenomena. In heat transfer applications, very high heat fluxes are observed, resulting in fast heating or cooling of the fluids. Other advantages of using microdevices for heat transfer operations are compactness, simple manufacturing, high heat dissipation per volume unit and low amount of refrigerant fluid requirements (Kim & Mudawar, 2013a). In mass transfer applications, the main advantage is obtained from the diffusion path reduction, improving the molecular diffusion. Smart device of the design can also induce split and recombination of streams and vortex generation, even at relative low Reynolds numbers, enhancing the mass transfer rates due to chaotic advection mechanism. The hydraulic diameter decrease from macro to micro scale was experimentally demonstrated as an efficient way to improve the heat and mass transfer. However, the pressure drop restriction bounds the lower dimension of the channel. Accordingly, the microdevice design should always take into account the heat transfer jointly with the hydraulic characteristics (Wang & Wang, 2014). The microdevices applications in heat and mass transfer include cooling of microelectronic devices and high power laser diode arrays, chemical reactors, fuel cells and micro-heat exchangers (Santana, Sanchez, & Taranto, 2017a; Santana, Tortola, Silva Jr., & Taranto, 2017b; Deng, Wan, Zhang, & Chu, 2017; Bakhtiary-Davijany, Hayer, Phanam, Myrstad, Venvik, Pfeifer, & Holmen, 2011; Fazeli & Behnam, 2010).
Flow boiling in micro- and milli-channels (mini-channels) can present distinct behavior concerning the well-established observed from macroscale due to the changes in the dominant forces (Wang & Wang, 2014). Several studies were carried out in the investigation of boiling flow performance (vapor quality) dependence on the main operating parameters: wall heat flux, degree of feedstream subcooling, pressure and mass velocity (Mahmoud & Karayiannis, 2013; Sun & Mishima, 2009; Kandlikar & Balasubramanian, 2004; Huo, Chen, Tian, & Karayiannis, 2004; Kew & Cornell, 1996; Liu & Winterton, 1991). Most of the studies were performed using pure substances, once the heat transfer parameters of the boiling flow were the main goal of these studies. In contrast, the use of microchannel devices for simultaneous heat and mass transfer in mixtures, i.e., evaporation of the species A from a binary or multicomponent mixture (e.g., excess alcohol from a biodiesel synthesis downstream) still requires further investigation. In this context, the present chapter aims to describe the main characteristics of boiling flow in micro- and milli-channels and to present experimental and numerical results from a microchannel heat exchanger with phase change (micro evaporator) applied to the recovery of excess ethanol from biodiesel.
Key Terms in this Chapter
Vapor Quality: Ratio of the vapor mass to the vapor-liquid mixture mass.
Eulerian Multiphase Approach: Mathematical framework treating all phases of the system as a continuous and interpenetrating phase.
Microevaporator: Microdevice for heat transfer service promoting liquid-to-vapor phase change.
Boiling Heat Transfer Coefficient: Global heat transfer coefficient given by the contribution of nucleate boiling and convective boiling mechanisms.
Thermal-Hydraulic Performance: Performance of the equipment based on the balance between the heat transfer coefficient (or the heat transfer capacity) and the pressure drop.
Subcooling Degree: Temperature difference between the liquid and the liquid saturation temperature.
Phase Holdup: Volumetric fraction occupied by a phase in a multiphase system (i.e., the ratio of the volume of the phase to the total volume of mixture).
Conjugate Heat Transfer Flow: Fluid flow with simultaneous heat and momentum transfer (i.e., fluid flow with heating or cooling).