Investigation of one-directional and bi-directional flows across staggered tube banks heat exchanger

One-directional and bi-directional flow conditions are different in terms of the direction of fluid flows. One-directional flow condition is when the fluid flows only in one direction. Meanwhile, bi-directional flow condition happens when the fluid flows in back and forth directions and this has been found in applications such as thermoacoustics. Thermoacoustics offer green technology for refrigeration and power cycles. The technology is appealing, but the lack of understanding on fluid dynamics and heat transfer behaviours of flow inside the system lead to the ambiguous use of equations from the well-known one-directional flow during the design stage and the impact of such estimation becomes evident as the flow in the real system becomes more complex. For this reason, experimental investigations that are supplemented by computational fluid dynamics modelling results are carried out with a focus on the heat exchanger. A staggered tube-banks heat exchanger was tested. A thermoacoustic’s standing wave rig with different type of flow inducers is used to create the one-directional and the bi-directional flows for the experiments. Results of velocity and temperature were recorded at the upstream and downstream locations of the staggered tube banks heat exchanger. The frequency of the bi-directional flow was set based on the resonance frequency of 14.2 Hz. Results indicate that temperature and velocity changes with respect to the change of flow amplitude are different between the one-directional and bi-directional flow conditions and the differences are in the range of 77 percent and 59.5 percent, respectively. A two-dimensional model of staggered tube banks heat exchanger is solved using a commercial software Ansys Fluent 16.0. The flow conditions were solved for the range of Reynolds between 270 and 1700 using SST k- turbulence models and the temperature contour, vorticity contour and the velocity vectors were discussed based on the results of the validated models. A similar trend of Nusselt number changes with the change of Reynolds number is seen between the experimental and numerical works of the one-directional flow and the bi-directional flow conditions, with errors of amplitude expected to be due to the limitations of experimental apparatus as well as the simplifications and assumptions made for the computational fluid dynamics works. Nevertheless, most recorded differences fell within the experimental uncertainty values of 2.8 to 5.8. The computational fluid dynamics models provided insight into the visualization of flow in area that could not be seen experimentally. Evidently, the bi-directional flow conditions are different compared to the one-directional flow condition by 65.85 percent at Reynolds number of 1300. Therefore, the use of the well-established one-directional flow equation on bi-directional flow conditions should be avoided should an accurate result is needed in the design stage of the future thermoacoustic energy systems

Temperature And Velocity Changes Across Tube Banks In One-Directional And Bi-Directional Flow Conditions

The back-and-forth movement of flow in oscillatory flow condition that can be found in blood flow, thermoacoustic energy system and ocean wave can be categorized as bi-directional flow condition and heat transfer in this flow condition is not well understood. This paper reports an experimental investigation that compares temperature and velocity values between the onedirectional (the usual flow condition) and the bi-directional flow conditions. The experiment was done using thermoacoustic’s standing wave rig with two different drivers to drive the one-directional flow and bi-directional flow conditionsin the test rig. Results, that were recorded using piezoresistive pressure sensor, type-K thermocouple and hotwire anemometer, indicate that care should be exercised when calculating heat transfer in bi-directional flow conditions as the temperature and velocity changes are different compared to the one-directional flow condition. Differences were recorded to be within the range of 77% for temperature and 59.5% for velocity, presumably due to the different behavior of forced and natural convection effect as flow conditions change

Experimental and numerical studies of one-directional and bi-directional flow conditions across tube banks heat exchanger

Bi-directional flow condition imposes different fluid dynamics and temperature changes compared to that of the usual one-directional flow condition. Bi-directional flow can be found in applications like thermoacoustic systems that offer a green technology for at least two major applications: refrigeration and power production. The technology is appealing as an alternative to traditional systems as it offers the replacement for the use of harmful working media and exhausted resources with the use of inert gaseous with relatively fewer moving mechanisms. As the fluid dynamics and heat transfer of bi-directional flow in a thermoacoustic working environment is less known, it is difficult to estimate losses and gain, especially during the design stage. This paper reveals the differences to be expected in the behaviour of flow and heat transfer through experimental as well as Computational Fluid Dynamics (CFD) results of one-directional and bi-directional flow conditions. Two different drivers were used to create the two different flow conditions: a loudspeaker for the bi-directional flow and a centrifugal blower for the one-directional flow. Both conditions were monitored based on flow amplitude that is calibrated between the two drivers. Results of velocity, temperature and, vorticity are recorded for Reynolds number that ranges between 270 and 1700. Analyses are supplemented with data from validated two-dimensional computational fluid dynamics models that were solved using the Shear-Stress-Transport (SST) k-ω turbulence model with second-order accuracy for all equations. Interesting features of differences in temperature and velocity changes between the one-directional and the bi-directional flows are reported. The temperature and velocity at upstream and downstream locations of the tube banks heat exchanger are almost the same for bi-directional cases but are significantly different when a one-directional flow is flowing over the heated tubes. In addition, the interplay between natural and forced convections is seen to affect the results that were recorded for the two flow conditions. The presence of thermally developing and fully developed regions is also discussed. The results indicate that the heat transfer behaviour of bi-directional flow is not the same as in the one-directional flow and the future calculation for heat transfer for bi-directional flow conditions of thermoacoustic must be carefully done with consideration of changes of flow conditions between the one-directional and the bi-directional flow conditions so that error could be minimized in the evaluation of the system’s performance

Uncertainty analysis of thermal fluid measurements for Bi-directional flow condition across tube banks

The uncertainty analysis for experimental investigation of bi-directional flow conditions of thermoacoustics is presented. The experimental rig used for loudspeaker as a flow inducer to provide acoustical flow across tube banks that is placed inside a standing wave resonator. The measured velocity and temperature changes within the vicinity of the tube banks are presented along with the uncertainty values. The standard deviation for velocity and temperature data shows that data varies with maximum deviation of 0.14 m/s and 5.78 °C, respectively. The results show that a good repeatability was obtained during the experiments which indicates that a reliable thermal-fluid measurement of bi-directional flow condition was achieved