The apparatus used to implement the

The apparatus used to implement the exchange of heat between
two fluids that are at different temperatures while keeping them
from mixing with each other is termed a heat exchanger. They are
used in a great amount of applications, such as heating and air
conditioning systems, chemical processing, power production and
aeronautical applications. The weight and size of heat exchangers
used in space or aeronautical applications are very significant parameters,
and in these cases, costs of heat exchanger production are
enumerated as a secondary consideration. However, heat exchangers
are designed to obtain maximum heat transfer, low
pressure drop, high effectiveness, minimum volume and weight,
and also low cost. Several techniques have been employed for these
purposes. Generally, these methods can be categorized into three
principal types: (1) using of nanofluids (2) coarsening heat
exchanger surfaces and (3) inserting fluid turbulators. Coarsening
surfaces can improve the thermal performance of heat exchanger
by increasing fluid mixing and turbulence level of the fluid flow.
Corrugated tubes can be introduced as one of the coarsening surface
methods and they can be classified into two main types: inward
concave corrugated tubes and outward convex corrugated
tubes. Both of them are studied in present paper.
Numerous experiments on thermal and frictional characteristics
of corrugated tubes have been carried out. Many researchers have
investigated the heat transfer and flow characteristics of fluid in a
single horizontal corrugated tube, and some of the researchers
focused on the using of corrugated tube as the inner tube of a
double pipe heat exchanger.
Publications on characteristics of a single corrugated tube and
double pipe heat exchangers made of corrugated tubes are summarized
chronologically as follows. Rainieri and Pagliarini [1]
experimentally studied on convective heat transfer in the entry
region of corrugated tubes at different pitch ratios for Reynolds
number range between 90 and 800. Ethylene glycol was employed
as the working fluid. Their findings showed that for Re > 200 the
helical corrugation significantly induces swirl component. Besides,
the results showed that in the turbulent flow the Nusselt number is
independent on the corrugation shape. Ahn [2] studied experimentally
on heat transfer in the annuli with corrugated inner tubes
for water flow rate in 700 < Re < 13,000 regime. Vicente et al. [3]
performed an experiment about heat transfer and isothermal
pressure drop in corrugated tubes for laminar and transition flow.