Jiajun Xu

By Mehdi Kabirnajafi and Jiajun Xu

Vapor quality is one of the crucial parameters substantially affecting the flow boiling heat transfer coefficient. Hence, the reliability and accuracy of vapor quality measurements is of a great significance to accurately investigating the effect of vapor quality on the local flow boiling heat transfer coefficients. In the present study, various experimental approaches are represented to measure and control local vapor quality for flow boiling tests. Experimental approaches are classified based on the type of thermal boundary conditions imposed on the tube wall, that is, known constant wall heat flux and constant wall temperature (unknown variable wall heat flux). In addition, in-situ techniques are also investigated to measure local vapor quality regardless of the governing thermal boundary conditions within two-phase flow experiments. Finally, the experimental methodologies are compared based on their level of reliability and accuracy in measurement, costliness and affordability, and simplicity in execution to address their potential merits and demerits.

Part of the book: Heat Transfer

By Jaime Rios, Mehdi Kabirnajafi, Takele Gameda, Raid Mohammed and Jiajun Xu

The present study experimentally and numerically investigates the flow and heat transfer characteristics of a novel nanostructured heat transfer fluid, namely, ethanol/polyalphaolefin nanoemulsion, inside a conventionally manufactured minichannel of circular cross section and a microchannel heat exchanger of rectangular cross section manufactured additively using the Direct Metal Laser Sintering (DMLS) process. The experiments were conducted for single-phase flow of pure polyalphaolefin (PAO) and ethanol/PAO nanoemulsion fluids with two ethanol concentrations of 4 wt% and 8 wt% as well as for two-phase flow boiling of nanoemulsion fluids to study the effect of ethanol nanodroplets on the convective flow and heat transfer characteristics. Furthermore, the effects of flow regime of the working fluids on the heat transfer performance for both the minichannel and microchannel heat exchangers were examined within the laminar and transitional flow regimes. It was found that the ethanol/PAO nanoemulsion fluids can improve convective heat transfer compared to that of the pure PAO base fluid under both single- and two-phase flow regimes. While the concentration of nanoemulsion fluids did not reflect a remarkable distinction in single-phase heat transfer performance within the laminar regime, a significant heat transfer enhancement was observed using the nanoemulsion fluids upon entering the transitional flow regime. The heat transfer enhancement at higher concentrations of nanoemulsion within the transitional regime is mainly attributed to the enhanced interaction and interfacial thermal transport between ethanol nanodroplets and PAO base fluid. For two-phase flow boiling, heat transfer coefficients of ethanol/PAO nanoemulsion fluids were further enhanced when the ethanol nanodroplets underwent phase change. A comparative study on the flow and heat transfer characteristics was also implemented between the traditionally fabricated minichannel and additively manufactured microchannel of similar dimensions using the same working fluid of pure PAO and the same operating conditions. The results revealed that although the DMLS fabricated microchannel posed a higher pressure loss, a substantial heat transfer enhancement was achieved as compared to the minichannel heat exchanger tested under the same conditions. The non-post processed surface of the DMLS manufactured microchannel is likely to be the main contributor to the augmented heat transfer performance. Further studies are required to fully appreciate the possible mechanisms behind this phenomenon as well as the convective heat transfer properties of nanoemulsion fluids.

Part of the book: Heat Transfer

By Bao Yang and Jiajun Xu

Part of the book: Energy Consumption, Conversion, Storage, and Efficiency

By Jiajun Xu, Faridreza Attarzadeh and Tanjee Afreen

As the adaptation of lithium (Li) ion batteries (LIBs) in energy storage systems is becoming more prevalent by the day, the issue of safe and environmentally responsible design, installation, and operation of these batteries is posing a rapidly growing challenge. It is imperative to develop realistic multi-physics and multi-scale models that are useful not only for analyzing the thermal runaway (TR) events at the single-cell level but also for modular LIB designs. This needs to be accompanied by the development of easier-to-follow empirical rules and straightforward analytical models as our knowledge of TR events grows over time. The unpredictable nature of TR events and the grave fire and explosion dangers that are particularly associated with violent TR events at the modular level require employing large-scale real-time evaluation of these events as well. Although more innovative battery health indicators are being developed and employed, it is still very challenging to arrest catastrophic TR events in time. The review herein seeks to explore advanced modeling and experimental approaches holistically. The challenges and possibilities of different active and passive thermal management strategies are also critically elaborated for LIB modular designs.

Part of the book: Energy Consumption, Conversion, Storage, and Efficiency