**1. Introduction**

The use of annular passages in application engineering extends through such areas as heat exchangers, gas turbines, cooling of nuclear reactor and some operation industries, cementing operations, formation fracturing, and flow of lubricants in journal bearing and pressure bushings, as well as those applications found in the form of horizontal or vertical directions. Therefore, many researchers have studied fluid flow and heat transfer in annular passages in horizontal and vertical directions for different types of fluid used in varied applications. Thus, investigations that deal with different fluids, for example air, water, oil, gas, etc., are included in this study. Regarding annular passages, there are two types; in concentric annular passages, the position of the inner pipe is in the center of the outer pipe/passage, and in eccentric annular, the position of the inner pipe is not in the center of the outer passage.

Enhancement of heat transfer has been widely researched with different techniques in the last decades. Most researchers have studied the effect of changing the feature of geometry on heat transfer rate. The flow through an axisymmetric sudden expansion or contraction, over backward-facing or forward-facing steps and through ribbed channels, creates separation flow.

There are many experimental and numerical studies that have investigated the effect of separation flow on performance of heat transfer, using different configurations and boundary conditions. Most of these investigations were carried out for separation air flow, while a few were carried out for separation liquid flow in sudden expansion. In the last decade, researchers have used nanofluid in their studies to improve augmentation of heat transfer. Studies on heat transfer to nanofluid flow in sudden expansion, or over backward-facing and forward-facing steps, are very limited for the laminar range, and most have been numerical, the turbulent range of nanofluid flow has not yet been investigated. The main efforts toward studying

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separation flow were carried out in the late 1950s. All of these efforts were performed exper‐ imentally using many flow visualization techniques, and they deal exclusively with turbulent or laminar flows. This chapter covers most of the investigations that have studied heat transfer and pressure drop of separation flow with fluid and nanofluid, in sudden expansion and backward-facing and forward-facing steps, as well as heat transfer and flow in annular pipes.

### **2. Nanofluid**

Nanofluid is a mix of a base fluid and nanometer-sized particles called nanoparticles. The base fluid is commonly water, oil, and ethylene glycol, while there are different types of nanopar‐ ticles, including metals, carbides, oxides, and carbon nanotubes. There are several parameters that affect the performance of nanofluid, including the size and shape of the nanoparticles, concentration, base fluid, and if the nanoparticle type is metal or non-metal. For example, nanoparticle size has a significant impact on thermal conductivity; the small size of the nanoparticle leads to an increase in surface area, and therefore researchers have employed different types of nanofluids in different geometries to reach augmentation of heat transfer.

Nanofluid can be prepared by two methods. In summary, the first method creates nanoparti‐ cles with chemical or physical processes (e.g., evaporation and inert-gas condensation processing), and then disperses them into a host fluid. The second method includes production and dispersal of the nanoparticles directly into a host fluid.
