| dc.description.abstract | The purpose of this investigation is to characterise the fluid flow and heat transfer behaviour of a dual jet
comprising a wall jet, discharged immediately parallel to a solid boundary, flowing alongside a parallel jet
whose exit is offset from the solid boundary by some distance, i.e., an offset jet. Despite the widespread
industrial applications of dual jet flows, which include turbine blade shielding, heat exchangers, fuel injection
systems, wastewater evacuation process, air-conditioning, and noise suppression technology, the flow and heat
transfer characteristics of a wall-bounded dual jet remains severely underrepresented in the published literature.
As a result, the heat transfer capabilities and driving flow mechanisms associated with dual jet flows are not
yet fully understood, meaning they cannot be optimised towards specific applications.
Most notably, the lack of verified, repeatable experimental flow and heat transfer measurements in the
published literature has led to significant discrepancies across the published numerical findings, which furthers
the requirement for the provision of a suitable means of validation with simulating dual jet flows. The present
study is thus concerned with characterising the transfer of heat to a dual jet flow for varying Reynolds number,
offset ratio and velocity ratio through experimental means, as well as acquiring an understanding of the
underlying flow phenomena which drive this heat transfer behaviour. The findings of the experimental
investigation are hence used to develop a validated numerical dual jet model and improve on the dual jet
numerical modelling techniques available in the literature.
For the experimental investigation, air is adopted as the working fluid and each jet Reynolds number is varied
in the range 5500 ≤ 𝑅𝑒 ≤ 12000, while the offset ratio and velocity ratio are varied in the ranges 1 ≤ 𝑂𝑅 ≤
7 and 0.5 ≤ 𝑉𝑟 ≤ 2, respectively. The initial heat transfer study subjects the airflow to a constant heat flux
generation of 1670 𝑊/𝑚2
in the bounding wall and the surface Nusselt number distribution is measured using
infrared thermography techniques. The velocity field is subsequently captured using particle image velocimetry
for a more limited offset ratio range of 1 ≤ 𝑂𝑅 ≤ 3, while the range of 𝑅𝑒 and 𝑉𝑟
remain consistent.
The results reveal a distinct time-averaged local Nusselt number profile in the wall boundary, where the
magnitude and positions of a local 𝑁𝑢𝑥 minimum and maximum can readily be controlled through adjusting
𝑅𝑒, 𝑂𝑅 and 𝑉𝑟
. This is attributed to changes in the size of the recirculation zone, which ultimately dictates the
extent of the wall jet deflection and, subsequently, the thickness, length and streamwise position of a small
separation region induced adjacent to the solid wall boundary. Analysis of the surface-averaged Nusselt number
indicates a linear relationship between ̅𝑁̅̅̅𝑢 𝑥 and 𝑅𝑒, where ̅𝑁̅̅̅𝑢 𝑥 rises linearly with increasing jet Reynolds
number, while higher values of ̅𝑁̅̅̅𝑢 𝑥 are noted for lower 𝑂𝑅. The time-resolved flow data indicates the
occurrence of periodic vortex shedding, the frequency of which is dependent on 𝑅𝑒, 𝑂𝑅 and 𝑉𝑟
, however, a
limited range of unsteady 𝑉𝑟
values are identified.
The numerical investigation uses Ansys Fluent 2023 R2 to model an identical dual jet flow configuration by
means of the 𝛾 − 𝑅𝑒𝜃 transition model. The numerical data is acquired using a steady state solver for a limited
subset of the experimental range of jet parameters, i.e., 5500 ≤ 𝑅𝑒 ≤ 12000, 𝑉𝑟 = 1 and 𝑂𝑅 = 1 and 3. A
velocity inlet is applied at the entrance to each jet, where the associated velocity profile, turbulence intensity
and turbulent viscosity ratio are tuned appropriately with respect to the experimental data. The results reveal a
successful emulation of the experimental heat transfer findings, thus enabling a full validation of the numerical
model setup within the accepted uncertainty limits. The numerical model hence introduces significant
improvements to dual jet numerical modelling compared to that available in the published literature, despite a
slightly slower rate of jet development, where the resulting heat transfer behaviour can be considered more
reliable and better aligned with physical reality. | en |
