Two-phase Flow Heat Exchangers Research Articles

Robust and general predictive models for condensation heat transfer inside conventional and mini/micro channel heat exchangers

To design, scale-up, and optimize the two-phase flow heat exchangers, accurate information about the heat transfer coefficient (HTC) is required. Here, an extensive database including 11,128 experimental data points from 80 sources, covering 37 condensing fluids over a wide range of operating conditions, were gathered for developing general and accurate models to forecast the HTC in mini/micro and conventional channel heat exchangers. The conventional intelligent approaches, namely, gaussian process regression (GPR), radial basis function (RBF), and hybrid radial basis function (HRBF) and also the least square fitting method (LSFM) were utilized for development of the new models. Firstly, the gathered data were used in evaluation of earlier two-phase HTC models, and these models showed average absolute relative error (AARE) values higher than 29%. Thus, more accurate models are necessary for these systems. Based on Pearson’s correlation analysis, eight dimensionless groups were selected as the most important input parameters. The GPR model showed the best results in estimation of Nu number for the test process with AARE of 4.50%. However, RBF and HRBF models estimated the HTC with AARE values 19.41% and 24.36% for testing data points, respectively. In addition, a general explicit correlation was developed by the least square fitting method (LSFM) for estimating the HTC, with acceptable results at an AARE of 23.40% for all analyzed data. The performance of the new general correlation, GPR and the previous models were assessed for different channel sizes, flow regimes, fluid types, and operating conditions. The new models and well-known earlier ones were examined with about 372 additional data samples from four new independent sources. The proposed models showed excellent results for prediction of HTC. Finally, a sensitivity analysis was studied based on the experimental data and the outcomes of the LSFM.

Flow boiling heat transfer and pressure drop characteristics of Isobutane in horizontal channels with twisted tapes

Using twisted tapes as a passive method for heat transfer improvement in a two-phase flow heat exchanger is experimentally studied. The test evaporator is a copper channel with a length of 1000 mm and an internal diameter of 8.1 mm which is installed horizontally. Three twisted tapes with twist ratios of 4, 10, and 15 are used at refrigerant vapor qualities in the range of 0.1–0.8 and refrigerant mass velocities between 160-350 kgm−2s−1. The natural refrigerant Isobutane (R600a) is chosen as the working fluid because it is environmentally friendly. According to the experiments, installing twisted tapes inside the channel augments both heat transfer rate and pressure drops over the plain channel. It is also observed that for both plain and twisted tape inserted channels, the values of heat transfer coefficients and pressure losses grow by giving rise to the refrigerant mass velocity and vapor quality. Results showed that the system performance factor varied between 0.44–1.09 offering that using twisted tapes as a turbulator is beneficial under specific operating conditions. The empirical data showed that there is an optimum value of the working fluid mass velocity at which the performance of twisted tape inserted channels is higher.

Condensation heat transfer and pressure drop characteristics of Isobutane in horizontal channels with twisted tape inserts

• Using twisted tapes augmented the heat transfer and pressure drops. • The heat transfer rate enhanced by giving rise to vapor quality and mass flux. • The pressure losses increased by giving rise to vapor quality and mass flux. • The performance factor varied between 0.39 to 1.05 using inserts. The enhancement of heat transfer rate in heat exchangers has been considered a major concern. In this research, the impacts of inserting twisted tapes in horizontal two-phase flow heat exchangers are discussed, and different values of vapor qualities (in the range of 0.1 to 0.7) and mass velocities (in the range of 119 to 367 kgm −2 s −1 ) are considered during forced convective condensation of R600a (Isobutane). The test case is a pipe made from copper with an inner diameter of 8.1 mm and a length of 1000 mm. Furthermore, three twisted tape inserts with various twist ratios (defined as the ratio of the twisted tape pitch to the test pipe inner diameter) of 4, 10, and 15 are used. The results illustrated that installing twisted tapes results in the increment of pressure drops and the rate of heat transfer in comparison to the smooth case. Furthermore, the pressure drops and heat transfer rates augment as the refrigerant mass velocity and vapor quality increase. Depending on the inserts type and operating conditions, the performance factor (a criterion to assess the performance modification compared to the primary test case) between 0.39 to 1.05 was obtained. It was also observed that there exists an optimum amount of the refrigerant mass velocity at which the performance factor is higher. Results showed that generally using twisted tapes in heat exchangers is not instrumental unless when the main concern is to improve the heat transfer rate or when the augmented power consumed by pump can be justified.

Heat transfer augmentation in two-phase flow heat exchanger using porous microstructures and a hydrophobic coating

In this study, we improved the thermal performance of a slightly inclined tube for a two-phase flow heat exchanger by means of surface modification techniques. The exchanger condenses pure steam inside the tube while boiling takes place outside the tube in a pool of saturated water. First, appropriate surface modification techniques for each boiling and condensation surface were separately investigated. An electroplating technique with hydrogen bubbles was utilized to create porous microstructures as cavities on a boiling surface, which remarkably promoted heterogeneous bubble nucleation and resulted in significant enhancement on average 107% in the boiling heat transfer coefficient. Hydrophobic thin films of Teflon were coated on a condensation surface, which considerably enhanced heat transfer on average 100% by promoting dropwise condensation. Secondly, the selected surface modification techniques were applied to outer boiling and inner condensation surfaces of a single-tube two-phase flow heat exchanger and remarkable improvement in heat transfer performance (>60%) due to the surface treatments was experimentally demonstrated.

A moving boundary model for two-phase flow heat exchanger incorporated with relative velocities between boundaries and fluid

Moving Boundary (MB) dynamic model is an appealing approach for investigation of advanced control schemes for two-phase flow heat exchanger. For the confusion of relative velocities between boundaries and fluid existing in the previous MB model, this paper presents a modified moving boundary model. The dynamic model incorporated with the relative velocities is derived from physical principles of mass and energy conservation. And the model is then implemented in a novel underwater HYDROX system to predict cyclic performance. The simulation results from discretized model using MATLAB language show that the oscillations which is known as “Chattering” have been suppressed.

A nonlinear reduced-order modeling method for dynamic two-phase flow heat exchanger simulations

This article presents a nonlinear reduced-order modeling method for evaporators in vapor compression systems. This method utilizes linearized reduction techniques to replace the fast dynamic modes of a system with nonlinear functions calculated from the full-order equations of slow dynamic modes. This process creates a nonlinear reduced-order model that is capable of matching full-order nonlinear model results without suffering from linearization error and simulates over 20 times faster than the full-order model. Simulation results are presented for the application of this reduction method to both moving-boundary and finite-control-volume evaporator modeling paradigms. A wide array of case studies are performed for various model input perturbations to illustrate this method's performance advantages. Simulation time step results are presented that show the reduced-order nonlinear model's capability of numerical time steps two orders of magnitude larger than the full-order nonlinear model, resulting in drastically reduced computational times. This reduced computational time allows the use of more heavily discretized models, increasing the feasibility of using more detailed and accurate models for real-time simulation and control.

A Singular Perturbation Approach for Modeling Differential-Algebraic Systems

Dynamic systems described by an implicit mixed set of Differential and Algebraic Equations (DAEs) are often encountered in control system modeling and analysis due to inherent constraints in the system. A key difficulty in control and simulation of DAE systems is that they are not expressed in an explicit state space representation. This paper describes a general approach based on singular perturbation analysis for adding fast dynamics to a system of DAEs so that they can be expressed in an explicit state space form. Conditions for asymptotic convergence and approximation methods are investigated. The approach is illustrated for a model of a two-phase flow heat exchanger.

A general dynamic simulation model for evaporators and condensers in refrigeration. Part I: moving-boundary formulation of two-phase flows with heat exchange: Modèle général dynamique pour évaporateurs et condenseurs frigorifiques. Partic I: Formulation des conditions aux limites variables de flux biphasiques avec échange de chaleur

A mathematical model describing the transient phenomena of two-phase flow heat exchangers is presented. Based on the one-dimensional partial-differential equations representing mass and energy conservation (leaving out momentum equations assuming pressure drops to be negligible) and a tube-wall energy equation, a simpler set of ordinary-differential equations is formulated by integrating separately over the three zones (liquid, two-phase, and vapor) generally present in a heat exchanger. Special attention is given to the study of large transients induced by on/off control of vapor-compression cooling systems.

Fin efficiency of extended surfaces in two-phase flow

Abstract Compact plane-fin heat exchangers and other extended surface heat exchanges are used in several two-phase flow applications, such as condensation, boiling, or moist-air cooling. Because of simplicity and lack of better information, designers often use fin efficiency formulas for two-phase flows that are derived based on uniform heat transfer coefficient. However, the heat transfer coefficient h on the fin surface under condensation or boiling may vary significantly compared to the single-phase forced convection attributable to the possible existence of different two-phase flow regimes on the fin surface, including the condition of partial dryout. Thus, the use of constant h fin efficiency formulas (of single-phase flow) for two-phase flow situation may result in serious errors. A critical assessment is made in this paper of the various methods available for calculating fin efficiency in practical fin configurations with condensation, moist air cooling, and boiling on the extended surfaces. Conditions under which the use of constant h can be made are identified. Based on the literature review, some specific design recommendations are made for the determination of the fin efficiency for two-phase flow heat exchanger applications.

Heat resistance of Talaromyces flavus ascospores as determined by a two phase slug flow heat exchanger

Heat resistance of Talaromyces flavus was determined using two methods. Standard thermal death time vials were used to determine the heat resistance of T. flavus ascospores, from 80 to 90 °C with a decimal reduction time D 90 of 6.2 min. A continuous two-phase slug flow heat exchanger System with heating, holding and cooling sections was used to determine the heat resistance from 90 to 100 °C. Inoculated heating medium was pumped through 1.85 mm ID tubing in ‘slugs’ separated by air bubbles to interrupt laminar flow. Varying pump speed, length of tubing in the heating bath and temperature allowed collection of data under varied heating conditions. D 90 with the slug flow heat exchanger system was 6.4 min, indicating the two methods were comparable, z values were 6.7 and 6.4 °C, respectively. The slug flow heat exchanger system permits rapid determination of heat resistance over a range of temperatures. Because of minimum come-up time the slug flow heat exchanger can be used at temperatures near the upper limit of heat resistance for the microorganism being tested.

Effects of Maldistribution of Flow on Heat Transfer Equipment Performance

Maldistribution of flow in heat exchangers may have dramatic consequences on thermal and mechanical performance. This article summarizes some of the insights and experience discussed during a recent symposium on this subject. The paper concludes that maldistribution of flow should not normally be a problem in single-phase heat exchangers where good engineering practice is pursued. However, two-phase flow heat exchangers still pose a formidable challenge if poor performance resulting from maldistribution of flow is to be avoided.

Advanced study institute on: Thermal-hydraulic fundamentals and design on two-phase flow Heat exchangers Torres Vedras, Porto Novo, Portugal, 6–17 july 1987

Advanced study institute on: Thermal-hydraulic fundamentals and design on two-phase flow Heat exchangers Torres Vedras, Porto Novo, Portugal, 6–17 july 1987

An Evaluation of the Effect of Twisted-Tape Swirl Generators in Two-Phase Flow Heat Exchangers

Correlations covering all modes of single- and two-phase heat transfer with twisted-tape swirl generators have been incorporated into a computer program that predicts the effect of twisted tapes on the heat duty in fluid-to-fluid once-through vapor generators. These Results are compared with those from axial flow once-through vapor generators operating at the same inlet conditions. Pumping power and pressure drop with and without swirl generators are compared. The correlations used satisfactorily predict the performance of both types of heat exchangers. Significant area, pressure drop, and pumping power reductions are possible using twisted-tape swirl generators; almost the entire area reduction is attributable to the suppression of the critical heat flux condition.

Pasteurization of salted whole egg inoculated with Arizona or Salmonella.

Recently, Arizona bacteria, close relatives of Salmonella, were recovered from salted whole egg that had been pasteurized by the presently recommended process of 63.3 degrees C (146 degrees F) for 3.5 min. Because of this and the fact that the heat resistance of Arizona in salted whole egg had not been determined, the present study was undertaken. Arizona or Salmonella, grown in Trypticase soy broth supplemented with 2% yeast extract in Fernbach flasks covered with aluminum foil over cotton and guaze at 35 degrees C with shaking at 176 rpm for about 96 h, were found to have the greatest degree of heat resistance. As expected, these cells, when inoculated into salted whole egg at 10(7) cells per ml, survived heating at 63.3 degrees C (146 degrees F) for 3.5 min in a two-phase slug flow heat exchanger. To consistently achieve a 7-log kill of typical Salmonella or Arizona, a treatment of 67 degrees C (152.6 degrees F) for 3.5 min was required. However, if a 7-log kill is mandatory, it remains to be determined whether this process affect the functional properties of this product.