The First Law of Thermodynamics: Control Volumes

 Here we will extend the conservation of energy to systems that involve mass flow across their boundaries, control volumes. Any arbitrary region in space can be selected as control volume. There are no concrete rules for the selection of control volumes. The boundary of control volume is called a control surface. 



Conservation of Mass Like energy, mass is a conserved property, and it cannot be created or destroyed. Mass and energy can be converted to each other according to Einstein’s formula: E = mc2 , where c is the speed of light. However, except for nuclear reactions, the conservation of mass principle holds for all processes.   For a control volume undergoing a process, the conservation of mass can be stated as: total mass entering CV – total mass leaving CV = net change in mass within CV mi me  mCV



The conservation of mass can also be expressed in the rate form: m m dm dt CV  i  e  /   The amount of mass flowing through a cross section per unit time is called the mass flow rate and is denoted by m°. The mass flow rate through a differential area dA is: dm°= ρVn dA where Vn is the velocity component normal to dA. Thus, the mass flow rate for the entire cross‐section is obtained by:



A process during which a fluid flows through a control volume steadily is called steady‐ state process. A large number of devices such as turbines, compressors, and nozzles operates under the same conditions for a long time and can be modeled (classified) as steady‐flow devices.   The term steady implies no change with time. The term uniform implies no change with location over a specified region. A steady flow is characterized by the following:  

Comments

Post a Comment

Popular posts from this blog

First Law of Thermodynamics?

 A special case of law of conservation of energy with particular reference to heat and work. • Definition: Heat and work are interconvertible. • When a system undergoes a thermodynamic cycle then the net heat supplied to the system from surroundings is equal to the net work done by the system on its surroundings. First law for a process When a system executes a process the change in stored energy of the system is numerically equal to the net heat interactions minus the net work interactions during the process.  dE =δQ-δW So there exists a property of a closed system such that a change in its value is equal to the difference between heat supplied and the work done during the change of state. The property, E is called energy of the system. The energy resides within the system and it increases or decreases with change of state. Further , δQ = dE + δW . First law for an isolated System The energy of an isolated system is always constant. An isolated system is one in which there is...
Read more
  what is heat transfer rule ? ANS  - according to heat transfer rule the heat always transfer high temperature to low temperature .          its mean ia body1 has temp 500kelvin and body 2 has 200 kelvin , when the body 1 and 2 is connected each other then body heat flow  body 1 to 2 . and this heat flow timing upto eqilibrium stat
Read more

what is Brinell hardness test ? How to perform in the lab?

Image
BRINELL  HARDNESS TEST:  - Hardness of  a  material is generally defined as Resistance  to the  permanent  indentation under static and  dynamic load.  When a material is required to  use  under direct static or dynamic loads, only indentation hardness test will be useful to find out resistance to indentation.  In Brinell hardness test, a steel ball of diameter (D) is forced under a load (F) on to  a surface of test specimen. Mean diameter (d) of indentation is measured after the  removal of the load (F).  Title:   Brinell hardness test.   Aim:   To determine the hardness of the given specimen using Brinell hardness test. Specimen and specimen Brinell h...
Read more