{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

# 42540_03 - CHAPTER 3 FLUID FLOW Fluid Properties 3.1 Basic...

This preview shows pages 1–2. Sign up to view the full content.

3.1 CHAPTER 3 FLUID FLOW Fluid Properties .............................................................................................................................. 3.1 Basic Relations of Fluid Dynamics ................................................................................................. 3.2 Basic Flow Processes ...................................................................................................................... 3.3 Flow Analysis .................................................................................................................................. 3.5 Noise in Fluid Flow ....................................................................................................................... 3.13 Symbols ......................................................................................................................................... 3.14 LOWING fluids in HVAC&R systems can transfer heat, mass, F and momentum. This chapter introduces the basics of fluid mechanics related to HVAC processes, reviews pertinent flow pro- cesses, and presents a general discussion of single-phase fluid flow analysis. FLUID PROPERTIES Solids and fluids react differently to shear stress: solids deform only a finite amount, whereas fluids deform continuously until the stress is removed. Both liquids and gases are fluids, although the natures of their molecular interactions differ strongly in both degree of compressibility and formation of a free surface (interface) in liq- uid. In general, liquids are considered incompressible fluids; gases may range from compressible to nearly incompressible . Liquids have unbalanced molecular cohesive forces at or near the surface (interface), so the liquid surface tends to contract and has properties similar to a stretched elastic membrane. A liquid surface, therefore, is under tension ( surface tension ). Fluid motion can be described by several simplified models. The simplest is the ideal-fluid model, which assumes that the fluid has no resistance to shearing. Ideal fluid flow analysis is well developed (e.g., Schlichting 1979), and may be valid for a wide range of appli- cations. Viscosity is a measure of a fluid’s resistance to shear. Viscous effects are taken into account by categorizing a fluid as either New- tonian or non-Newtonian. In Newtonian fluids , the rate of defor- mation is directly proportional to the shearing stress; most fluids in the HVAC industry (e.g., water, air, most refrigerants) can be treated as Newtonian. In non-Newtonian fluids , the relationship between the rate of deformation and shear stress is more complicated. Density The density U of a fluid is its mass per unit volume. The densities of air and water (Fox et al. 2004) at standard indoor conditions of 68°F and 14.696 psi (sea-level atmospheric pressure) are U water = 62.4 lb m /ft 3 U air = 0.0753 lb m /ft 3 Viscosity Viscosity is the resistance of adjacent fluid layers to shear. A classic example of shear is shown in Figure 1, where a fluid is between two parallel plates, each of area A separated by distance Y . The bottom plate is fixed and the top plate is moving, which induces a shearing force in the fluid. For a Newtonian fluid, the tangential force F per unit area required to slide one plate with velocity V par- allel to the other is proportional to V / Y : F / A = P ( V / Y ) (1) where the proportionality factor P is the absolute or dynamic vis- cosity of the fluid. The ratio of F to A is the shearing stress W , and V / Y is the lateral velocity gradient (Figure 1A). In complex flows, velocity and shear stress may vary across the flow field; this is expressed by (2) The velocity gradient associated with viscous shear for a simple case involving flow velocity in the x direction but of varying mag- nitude in the y direction is illustrated in Figure 1B.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

### Page1 / 14

42540_03 - CHAPTER 3 FLUID FLOW Fluid Properties 3.1 Basic...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online