Stability analysis and maldistribution control of two-phase flow in parallel evaporating channels

Abstract

Parallel channel heat transfer system is an attractive means to enhance heat transfer by increasing the total surface area in the heat exchanger. Uniform distribution of flow in the channels is an important operation attribute in order to avoid channel dryout and the subsequent hot spots and possible device failure. For the system operating in the single-phase regime, uniform flow distribution is a stable equilibrium. However, in the two-phase boiling regime, stable equilibrium bifurcates -the uniform distribution becomes unstable, and new stable or unstable non-uniform distributions, or maldistributions, equilibria emerge. As a result, parallel channel heat exchangers typically operate in the single phase regime to avoid such "maldistribution instability." This paper presents a stability analysis and active flow and temperature control of a parallel channel evaporator in a pumped two-phase boiling heat transfer loop. We show that for identical channels, the system is uncontrollable with the pump alone and unobservable from the overall flow rate measurement. A conventional solution involves introducing additional valve-induced pressure drop, but the drawback is a resulting higher pumping power. We present a new, somewhat surprising, result that if the channel characteristics are all distinct, the system actually becomes controllable from * Corresponding author: TieJun Zhang, Tel.: +1-518-276-2125, Fax: +1-518-276-4897, E-mail: tjzhrpi@gmail.com the pump and observable from the total loop flow rate. For non-identical parallel systems, we show a controller design approach and demonstrate its efficacy through a simulation example.

Predicting two-phase flow distribution and stability in systems with many parallel heated channels

“…This has been demonstrated in Ref. [25] by transient simulations. However, pump control alone is insufficient when two or more channels are identical, in which case control with inlet valves is needed [24].…”

“…This limits the heat flux that can be safely dissipated without inducing an extreme temperature rise in the heat source. Several remedies have been proposed to suppress two-phase flow maldistribution and other (parallel-channel) instabilities: inlet restrictions [3,11,16,17], reentrant cavities [18], diverging cross-sections [19], seed bubbles [20], increased system pressure [21], self-sustained high-frequency oscillations [22], and active control of pump and/or valves [23][24][25][26]. However, these measures may not effectively suppress maldistribution specifically, may be infeasible to implement in some applications, or may increase pressure drop.…”

“…The stability of parallel channel systems was studied by Zhang et al [24,25] by examining the linearized dynamic system equations. Their analysis of the system dynamics revealed an interesting feature.…”

Design of a microscale organic Rankine cycle for high-concentration photovoltaics waste thermal power generation

“…However, for refrigerants, the pressure-enthalpy (P-h) and For a given working fluid, increasing the system pres sure usually decreases the liquid-to-vapor density ratio and then alleviates the flow instabilities, such as Ledinegg flow instability in microchannels [20] and parallel-channel flow maldistribution [22]. This also implies that elevated pressure reduces the possibility to initiate premature critical heat flux conditions caused by compressible flow oscillations [21] and also enhances heat flux removal abilities.…”

Transient data for flow of evaporating fluid in parallel mini pipes and comparison with theoretical simulations

“…They also performed steady state experiments and compared the theoretical flow rate distribution with their data. Zhang et al (2011) presented a stability analysis and active flow and temperature control of a parallel channel evaporator. Taitel and Barnea (2011) presented a transient model for flow rate distribution in evaporating two * Corresponding author.…”