An Investigation of Two-Phase Flow Characteristics in Chevron-Style Flat Plate Heat Exchangers

The purpose of this study is to investigate the two-phase flow field characteristics in flat plate heat exchangers. The main objectives are the causes of plate wetting/drying, measurement of fluid flow field characteristics, and the effects of enhanced plate passageways. Three types of passageways are being investigated: smooth-walled passageways, chevron-style passageways, and embossed (bumpy) plate passageways. Pressure drop and flow distribution data are examined over a Reynolds number range of 5,000~30,000. Different passageway orientations (horizontal, upflow, and downflow) are investigated. This paper will present flow field results for chevron-style and smooth-walled heat exchanger passageways with air and alkylbenzene oils as working fluids. INTRODUCTION Plate heat exchangers (PHE) have been widely used in the chemical and process industry for many years. The advantage of PHE relative to their shell-to-tube counterparts was clearly described in the reviews from Kerner et al. [1], Shah and Focke [2], and Williams [3]. PHE were originally used almost exclusively in the dairy, brewing, and food industries due to the ease with which they may be disassembled and cleaned to meet hygienic requirements. In the last 20 years, however, plate heat exchangers have been introduced to the refrigeration and air conditioning systems as evaporators and condensers for their high efficiency and compactness. With the increasing interest in PHE, it is important to know how to predict their thermal performance and the associated pressure drop. Plate heat exchangers often consist of an array of stamped or corrugated plates clamped together in a frame. "Brazed" construction is also common where the plates are fused together into an assembly. Due to the great variety of possible corrugation patterns, it seems impossible to provide users with generalized thermal and hydraulic design equations. Most investigations on PHE have involved determining the friction factor and heat transfer coefficient experimentally by modifying correlations available for flat plate channels of the forms: Eighth International Refrigeration Conference at Nu =a* Reb* Pre f:::: d*Ree Purdue University, West Lafayette, IN, USAJuly 25-28, 2000 87 Where a, b, c, and d are constants. A summary of published heat transfer and friction factor pressure drop correlations can be found in the review from Talik et al. [4]. Each correlation is only valid for specific plate-type corrugations within a certain Reynolds number range. The fluids in refrigeration heat exchangers are two-phase flow. Most studies mentioned above were conducted on single-phase liquid-to-liquid conditions using water as the working fluid. The information available for PHE at two-phase flow conditions is much more limited. Yan et al. [5, 6] investigated the performance of PHE under evaporation and condensation conditions with refrigerant R -134a as the working fluid. Characteristics of the flow patterns in two-phase flow are recognized as one of the most significant factors affecting pressure drop, heat transfer, and refrigerant charge in refrigerant systems. Condensation research results from Dobson at al. [7] show that condensation heat transfer is strongly affected by the characteristics of the two-phase flow structure. Evaporation investigation results from Wattlet et al. [8] and Kattan et al. [9-11] very clearly illustrate the importance of flow regime characteristics on heat transfer predictions for evaporation. Once we understand the two-phase flow characteristics of the flow patterns in a PHE, heat transfer relations may be developed by modeling the thermal boundary layers in the fluids, as indicated by Martin [12]. Flow visualization provides us a direct method to visually explore the flow pattern between two parallel plates. Luo et al. [13] investigated the single-phase flow pattern in two corrugated ducts of different corrugation patterns by the dye-injection technique with water as test fluids. A transition Reynolds number for flow in the duct was 80 to 120 was found. Flow visualization was also used by Yan et al [5] to examine R-134a evaporation in a heat exchanger. In this paper, flow visualization is used to study the characteristics of the two-phase flow patterns in PHE. Different passageway orientations (horizontal, upflow, and downflow) are investigated. The flow pattern in smooth-walled heat exchanger passageways was also investigated as a research base against which to compare to effects of enhanced plate passageways. Initial results for chevron-style passageways with oil-air are presented in this paper. Current activities are directed toward refrigerant (R123) saturation conditions and refrigerant -oil conditions. EXPERIMENTAL APPARATUS AND PROCEDURE Figure 1 shows a schematic of the adiabatic, near-ambient pressure, two-phase flow visualization loop used in this study. A liquid pump is used to circulate the liquid phase. A blower is used to transport vapor through 3.8cm inner diameter copper tubes to the test section. The liquid and vapor mixes together before entering the test section. The two-phase mixture from the test section is separated and returns to the liquid reservoir and vapor blower, respectively. Liquid mass flow rate is measured by a mass flow meter. Vapor mass flow is determined by measuring pressure drop through a straight tube section that is before the injection of the liquid phase. The air mass flow rate is calibrated with a commercial air mass flow meter. Vapor mass flow rate agreement between the mass flow meter and the tube pressure drop measurements is within 5 percent. The liquid/vapor mass flow rate and the mixture quality in Eighth International Refrigeration Conference at Purdue University, West Lafayette, IN, USAJuly 25-28,2000 88 test section can be controlled by adjusting by-pass valves and shut-off valves on the liquid/vapor lines. By changing the status of the shut-off valves, we can realize three different orientations: horizontal, vertical up, vertical down flow in the loop. Thermal couples are attached to the liquid/vapor lines and test section to measure the temperatures along the loop.