Does Annular Flow Boiling Inside a Tube improve Heat Exchanger Performance?
Vast literature is available about two-phase flow boiling in horizontal round tubes in heat exchangers. Several studies on flow pattern and mapping are also available. However, limited literature is available about two-phase flow in small cylindrical annular passages.
A detailed experimental study was conducted on a 19 mm diameter dimpled enhanced tube to evaluate the in-tube two phase heat transfer and pressure drop performance in an annular section created between the enhanced tube and a solid round PVC rod as shown in Figure 1. The purpose of the study was to understand the effect of forced early transition to annular flow on the pressure drop and heat transfer coefficient. The refrigerant studied was R-134a at a saturation temperature of 5°C, heat flux range 2.5 to 15 kW/m2, mass flux from 80 to 200 kg/m2-s and inlet vapor quality of 0.12 to 0.72. Flow pattern and pressure drop results were obtained under adiabatic conditions. The enhanced tube with a rod exhibited three times higher heat transfer performance versus same size smooth empty tube. The pressure drop enhancement was lower than the heat transfer enhancement at the lower mass fluxes which could help in designing smart DX heat exchanger (evaporator) especially with high latent heat refrigerants such as ammonia. The main objective of the study was to seek a way to avoid separated flow regime in the initial pass or passes of a typical horizontal DX heat exchanger by forcing the refrigerant to seek annular flow regime especially at the entrance to the first pass of the tube bundle.
The experimental setup comprised of refrigerant and water/ethylene-glycol circuits. The test loop is shown in Figure 2. Wall temperatures were measured at 4 equally spaced positions around the tube periphery for each of the 6 axial locations along the tube length.
Figure 1 Dimple tube with round insert
Figure 2. Schematic of the experimental facility
All experiments were conducted for a fixed inlet saturation temperature of 5°C. Flow pattern and pressure drop results were obtained for the test section under adiabatic conditions. The experimental results for pressure drop and heat transfer coefficient obtained for the dimpled tube were compared against predictions based on the methods for smooth tubes by Müller-Steinhagen and Heck (1986) and Liu and Winterton (1991), respectively.
Flow patterns observed
Figure 3 shows the flow pattern visualization upstream and downstream of the test section under adiabatic conditions. As expected, the presence of rod inside the tube caused an earlier transition from stratified flow (smooth and wavy) to annular flow. The following two distinct annular regimes were observed: (i) annular as usually defined in the literature characterized by a liquid film along the tube periphery; and (ii) annular with liquid waves, characterized by a thin liquid film along the tube periphery with intermittent fronts of liquid filling the entire annulus. This flow pattern was like an intermittent flow that kept a thin liquid film on the upper periphery of the tube.
Figure 3 Flow patterns at downstream and upstream of the test section for different mass flux and quality.
Figure 4 shows a comparison of the flow patterns as visualized in the present study and the flow pattern map of Wojtan et al. (2005) for smooth tubes based on the annulus hydraulic diameter. As shown in this figure, only annular flows at the test section outlet were visualized in the present study. It is thus clear that the PVC rod acts as an initiator of transition to annular flow at lower mass flux.
Figure 4 Comparison of flow patterns at the test section outlet with the flow pattern map of Wojtan et al. I- Intermittent; SW-stratified wave; S- stratified; A-Annular; D- Dry-out region.
Heat transfer enhancement and pressure drop penalty
Figure 5 shows heat transfer enhancement and pressure drop penalty compared to a smooth tube with the same average internal diameter. The heat transfer enhancement increased, and the pressure drop penalty decreased with decreasing mass flux. A threefold heat transfer enhancement was observed as compared to a smooth tube. However, it becomes negligible for a mass flux of 200 kg m-2 s-1. At reduced mass flux an increase in heat transfer enhancement was observed with reduced pressure drop penalty hence resulting in an overall better performance.
Thermal performance vs. Liu and Winterton prediction.
Pressure drop penalty vs. Müller-Steinhagen and Heck prediction.
Figure 5 Comparison of experimental results with prediction.
The results from this study can help design efficient heat exchangers such as DX evaporators. Shell and tube DX evaporators are designed with a multi-pass configuration. Proper circuiting is key to a stable operation. When the refrigerant expands while passing through an expansion valve it enters the first pass with lower quality. If the refrigerant is improperly distributed at the entrance the probability of two-phase separation increases. Once the flow is separated at an early stage, this penalty is further carried over to the next passes, therefore resulting in an underperforming evaporator and expansion valve hunting. One way to alleviate this potential operational problem is to use the concept of forced transition to annular flow as described in this study. While being concerned about the heat transfer performance, it is equally important to keep a close eye on the pressure drop too. Excessive pressure drop results in saturation temperature gradient and thus reducing the effective log-mean temperature (LMTD). The use of this concept could be of more significance in systems with higher latent heat fluid refrigerants such as ammonia or systems operating at lower saturation temperature where P-T curves for all refrigerants are flat. Proper refrigerant distribution is primarily a key to an optimum heat exchanger design.
Liu, Z., Winterton, R.H.S., 1991. A general correlation for saturated and subcooled flow boiling in tubes and annuli, based on a nucleate pool boiling equation. Int. J. Heat Mass Transf. 34 (11), 2759-2766.
Müller-Steinhagen, H., Heck, K., 1986. A simple friction pressure drop correlation for two-phase flow in pipes. Chem. Eng. Process. 20 (6), 297-308.
Wojtan, L., Ursenbacher, T., Thome, J.R., 2005. Investigation of flow boiling in horizontal tubes: Part I – A new diabatic two-phase flow pattern map. Int. J. Heat Mass Transf. 48 (14), 2955-2969.