Forced convection boiling in a stator–rotor–stator spinning disc reactor

Abstract

Boiling of a pure fluid inside the rotor-stator cavities of a stator-rotor-stator spinning disc reactor (srs-SDR) is studied, as a function of rotational velocity x, average temperature driving force DT and mass flow rate / m . The average boiling heat transfer coefficient h b increases a factor 3 by increasing x up to 105 rad s 21, independently of DT and / m . The performance of the srs-SDR, in terms of h b vs. specific energy input , is similar to tubular boiling, where pressure drop provides the energy input. The srs-SDR enables operation at > 10 5 Wm 23R , yielding values of h b not practically obtainable in passive evaporators, due to prohibitively high pressure drops required. Since h b is increased independently of the superficial vapor velocity, h b is not a function of / m and the local vapor fraction. Therefore, the srs-SDR enables a higher degree of control and flexibility of the boiling process, compared to passive flow boiling. V C 2016 American Institute of Chemical Engineers AIChE J, 00: 000-000, 2016 Keywords: evaporation, spinning disc reactor, heat transfer, intensification IntroductionForced convection boiling processes are widely encountered in the (chemical) process industry, e.g., in evaporative separations, in refrigeration cycles and in withdrawing heat of highly exothermic reactions.1,2 Intensification of boiling heat transfer reduces the required heat exchange area, which reduces the capital costs and the space requirements. Moreover, it allows the application of a lower temperature driving force DT, which is thermodynamically favorable and essential in utilization of low-grade heat streams using vapor recompression. Intensification of flow boiling is realized by increasing the boiling heat transfer coefficient h b , and by efficient removal of the formed vapor phase from the heat exchanging surface.The heat transfer in forced convection boiling occurs via two parallel mechanisms, viz. nucleate boiling and convective evaporation. [3][4][5][6][7] Both mechanisms contribute to the overall value of h b . Nucleate boiling is the formation of vapor bubbles at the heat exchanging surface, due to a high local superheating of the liquid. The formation of bubbles induces convection on a microscale, which leads to high values of h b , similar to pool boiling. 1,8,9 Convective evaporation is the macroscale fluid forced convection, with evaporation occurring at the liquid-vapor interface. Typically, at a high heat flux q 00 , obtained at a high DT, and a low vapor superficial velocity v G , h b is dominated by the nucleate boiling mechanism, resulting in a high value of h b . At a high vapor velocity (at a high mass flow rate / m or a high vapor fraction x G ), nucleate boiling is suppressed due to a more effective withdrawal of the local superheat by forced convection, 10,11 resulting in lower values of h b , compared to the nucleate boiling regime. 5,6 In this convective evaporation regime, h b increases with increasing turbulence in the liquid and decreasing distance between the heat exchanging ...

A new method for online monitoring of boiling process in mini‐channels

“…A channel with inner diameter of 1.0–3.0 mm is usually considered as a mini‐channel which is the transition between conventional scale channels and micro scale channels . With the development of modern industry, different kinds of mini‐channel heat exchangers become more and more important and are frequently encountered in many industrial fields, such as heat‐pipes, refrigeration evaporators/condensers, horizontal tubular evaporators, compact bean and tube heat exchangers, etc . For a mini‐channel heat exchanger, boiling is a common phenomenon.…”

“…For a mini‐channel heat exchanger, boiling is a common phenomenon. Since the heat performance and safety of a mini‐channel exchanger is strongly linked to the different boiling stages/states which reflect different bubble behaviors and heat transfer characteristics, the online monitoring of boiling process is of great importance for the operational heat transfer efficiency and stability of a mini‐channel heat exchanger …”

“…Some methods for monitoring of boiling process in mini‐channels have been developed in the past decades. The optical instrument (such as a high speed camera, an infrared camera, or a laser sensor) is the most commonly used approach to monitor the boiling process . Besides, temperature sensors (thermocouples) and pressure sensors are also adopted .…”

“…For an example, high‐speed cameras may be the most direct and frequent way to investigate boiling process. But they have the disadvantages of higher cost and are not suitable for the opaque fluids or nontransparent heat exchangers . Infrared cameras are also used commonly.…”

Rotor‐Stator Spinning Disc Reactor: Characterization of the Single‐Phase Stator‐Side Heat Transfer

“…Regarding the studies performed on heat transfer on rotorstator systems, the research varies greatly, as very different setups were examined from free discs [15,16], enclosed rotor-stator systems [17,18], (partially) open cavities [16,19,20] without superposed throughput [19][20][21] and with throughput [16,22,23]. Moreover, only little research has focused on closed rotor-stator cavities with superimposed throughput, yet [24][25][26][27].…”