Outflow without friction. Since water vapor is not an ideal gas, it is better to calculate its outflow not using analytical formulas, but using h, s-diagrams.
Let steam with initial parameters flow into a medium with pressure R 2. If the energy losses due to friction during the movement of water vapor through the channel and the heat transfer to the nozzle walls are negligible, then the outflow process proceeds at constant entropy and is depicted on h, s-vertical straight line diagram 1-2 .
The expiration rate is calculated by the formula:
where h 1 is determined at the intersection of lines p 1 and t 1, a h 2 is at the intersection of the vertical drawn from point 1 with the isobar R 2 (dot 2).
Figure 7.5 - Processes of equilibrium and non-equilibrium expansion of steam in the nozzle
If the enthalpy values are substituted into this formula in kJ/kg, then the outflow velocity (m/s) will take the form
.
Valid Expiration Process. In real conditions, due to friction of the flow against the walls of the channel, the outflow process turns out to be non-equilibrium, i.e., during the flow of gas, heat of friction is released and therefore the entropy of the working fluid increases.
In the figure, the nonequilibrium process of adiabatic expansion of vapor is conventionally depicted by a dashed line 1-2’.
At the same pressure drop 
actuated enthalpy difference 
gets less than 
, resulting in a decrease in the outflow velocity 
. Physically, this means that part of the kinetic energy of the flow is converted into heat due to friction, and the velocity head 
at the outlet of the nozzle is less than in the absence of friction. The loss in the nozzle apparatus of kinetic energy due to friction is expressed by the difference 
. The ratio of losses in the nozzle to the available heat loss is called the coefficient of energy loss in the nozzle 
:
The formula for calculating the actual velocity of an adiabatic nonequilibrium outflow is:
Coefficient 
called soonstnym
coefficient nozzles. Modern technology makes it possible to create well-shaped and machined nozzles that have 
Throttling of gases and vapors
It is known from experience that if an obstacle (local resistance) is encountered in the path of gas or vapor movement in the channel, partially blocking the cross section of the flow, then the pressure behind the obstacle is always less than in front of it. This process of decreasing pressure, resulting in neither an increase in kinetic energy nor technical work, is called throttling.
Figure 7.6 - Throttling of the working fluid in a porous partition
Consider the flow of the working fluid through a porous partition. Assuming that throttling occurs without heat exchange with the environment, we consider the change in the state of the working fluid when passing from the section I in section II.
,
where h 1,
h 2- enthalpy values in sections I and II. If the flow velocities before and after the porous partition are small enough so that 
, then 
So, with adiabatic throttling of the working fluid, its enthalpy remains constant, the pressure drops, and the volume increases.
Insofar as 
,
then from equality 
we get that
For ideal gases 
,
therefore, as a result of throttling, the temperature of an ideal gas remains constant, as a result of which .
When throttling a real gas, the temperature changes (the Joule-Thomson effect). As experience shows, the sign of temperature change ( 
for the same substance can be positive ( 
>0
),
gas is cooled during throttling, and negative ( 
<0
),
the gas heats up) in different areas of the state.
The state of the gas in which 
,
is called the point of inversion of the Joule-Thomson effect, and the temperature at which the effect changes sign is inversion temperature. For hydrogen it is -57°C, for helium it is -239°C (at atmospheric pressure).
Adiabatic throttling is used in the technique of obtaining low temperatures (below the inversion temperature) and liquefying gases. Naturally, the gas must be cooled to the inversion temperature in some other way.
The figure conditionally shows the change in parameters during throttling of an ideal gas and water vapor. The convention of the image is that non-equilibrium states cannot be depicted on a diagram, i.e., only the initial and final points can be depicted.
Figure 7.7 - Ideal gas throttling (a) and water vapor (b)
When throttling an ideal gas (Figure a) temperature, as already mentioned, does not change.
From h, s-diagram shows that during adiabatic throttling of boiling water, it turns into wet steam (the process 3 -4), moreover, the more the pressure drops, the more the temperature of the steam decreases and the degree of its dryness increases. When throttling high-pressure steam and slight overheating (process 5 -6) steam first passes into dry saturated, then into wet, then again into dry saturated and again into superheated, and its temperature eventually also decreases.
Throttling is a typical non-equilibrium process, as a result of which the entropy of the working fluid increases without heat supply. Like any nonequilibrium process, throttling results in a loss of available work. This is easy to see on the example of a steam engine. To obtain technical work with it, we have a ferry with parameters p 1 and t 1. The engine pressure is R 2 (if steam is released into the atmosphere, then R 2 = 0.1 MPa).
In the ideal case, the steam expansion in the engine is adiabatic and is depicted as h,
s-vertical line chart 1-2
between isobars
p1
(in our example 10 MPa) and p 2 (0.1 MPa). The technical work performed by the engine is equal to the difference between the enthalpies of the working fluid before and after the engine: 
. On the image b this work is represented by a line 1-2.
If the steam is pre-throttled in the valve, for example, up to 1 MPa, then its state in front of the engine is already characterized by the point 1’ . The expansion of steam in the engine will then go in a straight line 1"-2". As a result, the technical work of the engine, represented by the segment 1"-2", decreases. The stronger the steam is throttled, the greater the share of the available heat drop, represented by the segment 1-2, irretrievably lost. When throttled to pressure R 2 , equal in our case to 0.1 MPa (point 1’’ ), steam completely loses the ability to do work, because before the engine it has the same pressure as after it. Throttling is sometimes used to control (reduce) the power of heat engines. Of course, such regulation is uneconomical, since part of the work is irretrievably lost, but it is sometimes used due to its simplicity.
expiration process
With expiration processes, i.e. the movement of gas, vapor or liquid through channels of various profiles, is often encountered in technology. The main provisions of the theory of outflow are used in the calculations of various channels of thermal power plants: nozzle and working blades of turbines, control valves, flow nozzles, etc.
In technical thermodynamics, only the steady, stationary regime of the outflow is considered. In this mode, all thermal parameters and the outflow rate remain unchanged in time at any point in the channel. The patterns of outflow in an elementary stream trickle are transferred to the entire section of the channel. In this case, for each cross section of the channel, the values of thermal parameters and velocity averaged over the cross section are taken, i.e. the flow is considered as one-dimensional.
The main equations of the outflow process include the following:
Flow continuity or continuity equation for any channel section
where G is the mass flow rate in a given section of the channel, kg/s,
v - specific volume of gas in this section, m 3 / kg,
f - cross-sectional area of the channel, m 2,
c - gas velocity in the given section, m/s.
First law of thermodynamics for flow
l t, (2)
where h 1 and h 2 - gas enthalpy in 1 and 2 sections of the channel, kJ / kg,
q - heat supplied to the gas flow in the interval 1 and 2 of the channel sections, kJ / kg,
c 2 and c 1 - flow velocity in 2 and 1 sections of the channel, m/s,
l t is the technical work performed by the gas in the range of 1 and 2 sections of the channel, kJ/kg.
In this laboratory work, the process of gas outflow through the nozzle channel is considered. In the nozzle channel, the gas does not perform technical work ( l m = 0), and the process itself is fast, which causes the absence of heat exchange between the gas and the environment (q = 0). As a result, the expression for the first law of thermodynamics for an adiabatic gas flow through a nozzle has the form
. (3)
Based on expression (3), we obtain an equation for calculating the velocity in the outlet section of the nozzle
. (4)
In the experimental setup, the initial gas outflow rate is taken equal to zero (c 1 = 0), due to its very small value compared to the velocity in the nozzle exit section. The properties of a gas at atmospheric pressure or less obey the equation Pv=RT, and the adiabat of the reversible gas outflow process corresponds to the equation Pv K =const with a constant Poisson's ratio.
In accordance with the foregoing, the equation for the gas outflow rate at the outlet of the nozzle channel (4) can be represented by the expression
. (5)
In expression (5), the indices "o" indicate the parameters of the gas at the inlet to the nozzle, and the indices "k" - behind the nozzle.
Using the equations: flow continuity (1), the process of adiabatic gas outflow Pv K =const, and the equation for calculating the outflow velocity (5), we can obtain an expression for calculating the air flow through the nozzle
, (6)
where f 1 - the area of the outlet section of the nozzle.
The defining characteristic of the process of gas outflow through the nozzle is the value of the pressure ratio ε = P K / P O. At pressures behind the nozzle less than the critical pressure in the outlet section of the tapering nozzle or in the minimum section of the combined nozzle, the pressure remains constant and equal to the critical one. The critical pressure can be determined by the value of the critical pressure ratio ε KR = P KR / P O, which for gases is calculated by the formula
. (7)
Using the values of ε CR and P CR, it is possible to estimate the nature of the outflow process and choose the profile of the nozzle channel:
at ε > ε CR and P C > R CR the outflow is subcritical, the nozzle should be tapering;
for ε< ε КР и Р К < Р КР истечение сверхкритическое, сопло должно быть комбинированным с расширяющейся частью (сопло Лаваля);
for ε< ε КР и Р К < Р КР истечение через tapering the nozzle will be critical, the pressure in the outlet section of the nozzle will be critical, and the expansion of the gas from P KR to P K will occur outside the nozzle channel.
In the mode of critical outflow through a converging nozzle for all values of P K< Р КР давление и скорость в выходном сечении сопла будут критическими и неизменными, соответственно, и расход газа через сопло будет постоянный, соответствующий максимальной пропускной способности данного сопла при заданных Р О и Т О:
, (8)
, (9)
It is possible to increase the throughput of this nozzle only by increasing the pressure at its inlet. In this case, the critical pressure increases, which leads to a decrease in the volume in the outlet section of the nozzle, while the critical velocity remains unchanged, since it depends only on the initial temperature.
The real - irreversible process of gas outflow through the nozzle is characterized by the presence of friction, which leads to a shift of the process adiabat towards an increase in entropy. The irreversibility of the outflow process leads to an increase in the specific volume and enthalpy in a given nozzle section compared to a reversible outflow. In turn, an increase in these parameters leads to a decrease in the speed and flow rate in the actual outflow process compared to the ideal outflow.
The decrease in speed in the actual process of expiration characterizes the velocity coefficient of the nozzle φ:
φ \u003d c 1i /c 1. (ten)
The loss of available work due to the presence of friction in the real process of outflow characterizes the loss coefficient of the nozzle ξ:
ξ = l neg / l o \u003d (h ki -h k) / (h o -h k). (eleven)
Coefficients φ and ζ are determined experimentally. It suffices to define one of them, since they are interrelated, i.e. knowing one, you can determine the other by the formula
ξ \u003d 1 - φ 2. (12)
To determine the actual gas flow through the nozzle, the nozzle flow coefficient μ is used:
μ = G i /G theor, (13)
where G i and G theor are the actual and theoretical gas flow rates through the nozzle.
The coefficient μ is determined empirically. It allows, using the parameters of an ideal outflow process, to determine the actual gas flow through the nozzle:
. (14)
In turn, knowing the flow rate coefficient μ, one can calculate the coefficients φ and ξ for the gas flow through the nozzle. Having written expression (13) for one of the modes of gas flow through the nozzle, we obtain the relation
. (15)
The ratio of velocities and volumes in expression (15) can be expressed through the ratio of the absolute temperatures of the ideal and real outflow processes
Calculation of the expiration process using the h,s-diagram
| Parameter name | Meaning |
| Article subject: | Calculation of the expiration process using the h,s-diagram |
| Rubric (thematic category) | Technology |
Dividing the equation by pv, we find
(7.15)
Substituting the expression for , we get
(7.16)
Consider the movement of gas through a nozzle. Since it is designed to increase the flow rate, then dc>0 and y sign dF is determined by the ratio of the flow velocity to the speed of sound in a given section. If the flow rate is low ( c/a<1), то dF<0 (сопло суживается). В случае если же c/a>1, then dF>0,ᴛ.ᴇ. the nozzle should expand.
Figure 7.4 shows three possible relationships between the exhaust velocity with 2 and the speed of sound a at the outlet of the nozzle. With a pressure ratio 
the outflow velocity is less than the speed of sound in the outflowing medium. Inside the nozzle, the flow velocity is also everywhere less than the speed of sound. Therefore, the nozzle must be tapered over its entire length. The length of the nozzle affects only frictional losses, which are not considered here.
Figure 7.4 - Dependence of the nozzle shape on the outflow velocity:
a-
At a lower pressure behind the nozzle, you can get the mode shown in the figure. b. In this case, the speed at the exit of the nozzle is equal to the speed of sound in the outflowing medium. Inside, the nozzle should still taper (dF<0), and only in the exit section dF=0.
To obtain supersonic speed behind the nozzle, it is necessary to have a pressure below the critical pressure behind it (Fig. in). In this case, the nozzle is extremely important to be composed of two parts - tapering, where with<а, and expanding, where with>a. Such a combined nozzle was first used by the Swedish engineer K. G. Laval in the 80s of the last century to obtain supersonic steam velocities. Now Laval nozzles are used in jet engines of aircraft and rockets. The expansion angle should not exceed 10-12° so that there is no separation of the flow from the walls.
When gas flows out of such a nozzle into a medium with a pressure less than the critical one, critical pressure and velocity are established in the narrowest section of the nozzle. In the expanding nozzle, there is a further increase in velocity and, accordingly, a drop in the pressure of the outflowing gas to the pressure of the external environment.
Let us now consider the movement of gas through a diffuser - a channel in which the pressure increases due to a decrease in velocity pressure ( dc<0). Из уравнения * следует, что если c/a<1, то dF>0, i.e., if the gas velocity at the entrance to the channel is less than the speed of sound, then the diffuser should expand in the direction of gas movement, similarly to the flow of an incompressible liquid. If the gas velocity at the channel inlet is greater than the speed of sound ( c/a>1), then the diffuser should narrow (dF<0).
Outflow without friction. Since water vapor is not an ideal gas, it is better to calculate its outflow not using analytical formulas, but using h, s-diagrams.
Let steam with initial parameters flow into a medium with pressure R 2. If the energy losses due to friction during the movement of water vapor through the channel and the heat transfer to the nozzle walls are negligible, then the outflow process proceeds at a constant entropy and is depicted on h,s-vertical straight line diagram 1-2 .
The expiration rate is calculated by the formula:
where h 1 is determined at the intersection of lines p 1 and t 1, a h 2 is at the intersection of the vertical drawn from point 1 with the isobar R 2 (dot 2).
Figure 7.5 - Processes of equilibrium and non-equilibrium expansion of steam in the nozzle
If the values of enthalpies are substituted into this formula in kJ/kg, then the outflow velocity (m/s) will take the form
.
Valid Expiration Process. In real conditions, due to the friction of the flow against the walls of the channel, the outflow process turns out to be nonequilibrium, i.e., during the flow of gas, heat of friction is released and, in connection with this, the entropy of the working fluid increases.
In the figure, the nonequilibrium process of adiabatic expansion of vapor is conventionally depicted by a dashed line 1-2’.
At the same pressure difference, the actuated enthalpy difference 
is less than , due to which the outflow velocity also decreases. Physically, this means that part of the kinetic energy of the flow is converted into heat due to friction, and the velocity head at the outlet of the nozzle is less than in the absence of friction. The loss in the nozzle apparatus of kinetic energy due to friction is expressed by the difference 
. The ratio of losses in the nozzle to the available heat drop is commonly called the coefficient of energy loss in the nozzle:
The formula for calculating the actual velocity of an adiabatic nonequilibrium outflow is:
The coefficient is called speed coefficient nozzles. Modern technology makes it possible to create well-shaped and machined nozzles that have 
Calculation of the expiration process using the h,s-diagram - concept and types. Classification and features of the category "Calculation of the process of expiration using h,s-diagrams" 2017, 2018.
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The class of compressible liquids includes substances whose density varies depending on pressure and temperature. Gases (ideal and real) belong to the class of compressible liquids.
Potential work of a reversible adiabatic process of gas outflow from zero to the final state ( 0-2 ) is found from the relation
After substituting expression (256) into relation (248), we obtain a formula for calculating the gas outflow velocity in the outlet section of the nozzle
. (257)
To calculate the mass velocity of the gas according to the equation (), it is necessary to know the density of the gas in the exit section of the nozzle ( ), the value of which is determined from the adiabatic equation
. (258)
After a series of simple transformations, we obtain a relation for calculating the mass velocity of the gas in the outlet section of the nozzle
. (259)
Let us introduce into equation (259) the flow coefficient λ
(260)
and obtain the following relation for determining the mass velocity of the gas at the outlet of the nozzle
. (261)
An analysis of equation (259) for the mass flow rate shows that the gas velocity, changing depending on the pressure ratio in the process of outflow , vanishes twice - at p 2 / p 0 \u003d 1(no movement) and = 0 (outflow into vacuum, p 2 = 0). Consequently, the value of the mass velocity, according to Rolle's theorem, passes through an extremum (Fig. 23). The pressure ratio at which the mass flow rate becomes maximum () is called critical (), and the flow regime under this condition is called the critical flow regime.
Rice. 23. Dependence of the linear and mass velocities of the outflow
gas from the ratio of pressures in the process of expiration
To determine the characteristics of the critical flow regime, we denote by ψ the terms of equation (259) that depend on the quantity (the remaining terms depend only on the parameters of the initial state and the nature of the gas)
. (262)
We introduce into equation (262) an additional characteristic of the adiabatic expansion of the gas
. (263)
, (264)
. (265)
Obviously, the mass velocity will reach its maximum value at the same β cr, which is the function . The maximum condition for the function is
Based on relation (266), after transformation, we find the critical value of the characteristic of the adiabatic expansion of compressible liquids at the outflow () and the critical pressure ratio ():
. (268)
Substituting expression (267) into relation (257), we obtain an expression for calculating the critical linear velocity of the outflow
Considering that the following expression is true
, (270)
we obtain the following relations for calculating the critical linear velocity of the outflow:
; (271)
, (272)
where is the potential function of the compressible liquid in the nozzle section, where the critical outflow velocity (267), (270) is observed.
For a reversible adiabatic outflow of any compressible fluid, the critical linear velocity is equal to the local speed of sound in the given medium
. (273)
The value of the mass critical velocity of the outflow is determined from the relation
. (274)
Flow rate λcr in the critical expiration mode, it is found by substituting expressions (267) and (268) into relation (260)
The final expression for determining the flow rate in the critical flow mode λcr has the following form:
. (276)
Characteristics of the critical mode of the expiration of compressible liquids are given in Table. 3.
Table 3
Characteristics of the critical mode of the outflow of compressible liquids
For natural gases, the values of critical outflow parameters vary in the following ranges: τcr \u003d 0.85 - 0.90; β cr \u003d 0.53 - 0.56;
λcr \u003d 0.48 - 0.46.
The processes of gas and vapor outflow in converging nozzles or through holes in thin walls have a number of features. One of the features of the processes of gas and vapor outflow in narrowing nozzles or through holes in thin ones is the impossibility of realizing a supercritical flow regime.
On fig. 23 shows graphical dependences of the change in linear ( with) and mass ( u) the velocities of the outflow of incompressible liquids on the ratio of pressures in the process of outflow .
The area of the chart in which is called the area subcritical mode of expiration. In this area, the flow pressure in the outlet section of the nozzle () is equal to the pressure of the medium () into which the outflow occurs (), and with a decrease in the pressure of the medium (), there is an increase in the mass flow through the nozzle (), as well as linear () and mass () velocity flow in the exit section of the nozzle (Fig. 23).
After reaching the critical pressure ratio () comes critical expiration mode, at which the critical mode pressure is established at the nozzle outlet ( 
). This mode is characterized by critical values of mass flow rate (), linear () and mass () outflow velocity in the outlet section of the nozzle.
A further decrease in the pressure of the medium (), into which the substance flows out, does not lead to a decrease in pressure at the outlet of the nozzle, which remains unchanged and equal to the critical pressure (). This phenomenon is called "crisis flow". In the critical expiration mode, the flow velocity in the outlet section of the nozzle is set equal to the local sound velocity in the given medium (). Any perturbation propagates in the medium with the same speed (speed of sound). The critical exhaust velocity () established in the outlet section of the nozzle prevents the rarefaction wave from approaching this nozzle section, which predetermines the stabilization of the linear exhaust velocity at the level of the critical value even with a further decrease in the pressure of the medium. Under these expiration conditions ( 
) to increase the kinetic energy of the flow, not the entire available pressure drop (), but only part of it () is used.
Thus, when flowing through tapering nozzles and holes in thin walls, only two flow modes are possible - subcritical and critical. The process of outflow through converging nozzles and holes in thin walls is possible only if the following condition is met:
To ensure the supercritical flow regime, characterized by the condition (), it is necessary to supplement the converging nozzle with an expanding part, in the outlet section of which it is possible to achieve a pressure value below the critical one (). Such a combined nozzle is called a Laval nozzle.
In combined nozzles, the entire available pressure difference () can be used to increase the kinetic energy of the flow.
The transition from the expressions of theoretical exhaust velocities ( c 2 , u 2) to their real values () is carried out using the speed coefficients φ and expense μ determined empirically (values φ and μ less than one)
; 
. (278)
The processes of vapor outflow and, in particular, water vapor in a number of cases are calculated using h-s diagrams (Fig. 24).
Rice. 24. The process of the expiration of water vapor in h-s diagram
In a reversible adiabatic process, it follows from the first law of thermodynamics at that .
Using the equations of the first law of thermodynamics and the distribution of potential work (242) and taking into account that for short nozzles , we obtain the following relations.
Calculation of the expiration process using the h,s-diagram
Outflow without friction. Since water vapor is not an ideal gas, it is better to calculate its outflow not using analytical formulas, but using h, s-diagrams.
Let steam with initial parameters flow into a medium with pressure R 2. If the energy losses due to friction during the movement of water vapor through the channel and the heat transfer to the nozzle walls are negligible, then the outflow process proceeds at a constant entropy and is depicted on h,s-vertical straight line diagram 1-2 .
The expiration rate is calculated by the formula:
where h 1 is determined at the intersection of lines p 1 and t 1, a h 2 is at the intersection of the vertical drawn from point 1 with the isobar R 2 (dot 2).
Figure 7.5 - Processes of equilibrium and non-equilibrium expansion of steam in the nozzle
If the values of enthalpies are substituted into this formula in kJ/kg, then the outflow velocity (m/s) will take the form
.
Valid Expiration Process. In real conditions, due to the friction of the flow against the walls of the channel, the outflow process turns out to be nonequilibrium, i.e., during the flow of gas, heat of friction is released and, in connection with this, the entropy of the working fluid increases.
In the figure, the nonequilibrium process of adiabatic expansion of vapor is conventionally depicted by a dashed line 1-2’.
At the same pressure difference, the actuated enthalpy difference 
is less than , as a result of which the outflow velocity also decreases. Physically, this means that part of the kinetic energy of the flow is converted into heat due to friction, and the velocity head at the outlet of the nozzle is less than in the absence of friction. The loss in the nozzle apparatus of kinetic energy due to friction is expressed by the difference 
. The ratio of losses in the nozzle to the available heat drop is commonly called the coefficient of energy loss in the nozzle:
The formula for calculating the actual velocity of an adiabatic nonequilibrium outflow is:
The coefficient is called speed coefficient nozzles. Modern technology makes it possible to create well-shaped and machined nozzles that have 
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As the substantial switching frequency occurs the substantial losses in the transient modes occur as well, which involve heating of asynchronous motor that limits the amount of switching, breaking and reverse. These problems are very important at the operation of metal-cutting equipment, press, auxiliary lightning, where frequent switching is the condition of technology process. So the task is specified to choose minimal allowable duration of operation time, as the over temperature doesn`t ends... [read more]
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Die Lampe freundlich wieder brennt, Ach wenn in unsrer engen Zelle Als ein willkommner stiller Gast. So nimm nun auch von mir die Pflege, Durch Rennen und Springen ergetzt uns hast, Mein bestes Kissen geb ich dir. Lege dich hinter den Ofen nieder, Die Liebe Gottes regt sich nun. Es reget sich die Menschenliebe, Entschlafen sind nun wilde Triebe Die eine tiefe Nacht bedeckt, Mit ahnungsvollem, heil’gem Grauen In uns die... [read more]
MEPHISTOPHELES ALTMAYER Verlang ich auch das Maul recht voll. Denn wenn ich judizieren soll, Nur gebt nicht gar zu kleine Probenleise: Sie sind vom Rheine, wie ich spüre. MEPHISTOPHELES: Schafft einen Bohrer an (get / somewhere / a drill; anschaffen - acquire, buy, get, get; bohren - drill, drill)! BRANDER: Was soll mit dem geschehn... [read more]
Nicht ein Geschmeide, nicht ein Ring, Ich schielte neulich so hinein, Das Kesselchen herauszuheben. Du kannst die Freude bald erleben, Die herrliche Walpurgisnacht. So spukt mir schon durch alle Glieder Das an den Feuerleitern schleicht, Wie von dem Fenster dort der Sakristei Faust. Mephistopheles. FAUST:Aufwärts der Schein des Ew'gen Lämpchens flämmert Und schwach und schwächer seitwärts dämmert, ... [read more]
Say - Tell - Ask - Speak - Talk REPORTED SPEECH UNIT 19 Direct Speech gives the exact words someone said. We use inverted commas in Direct Speech. “It's a nice song,” he said. Reported Speech gives the exact meaning of what someone said but not the exact words. We do not use inverted commas in Reported Speech. He said it was a nice song. Say is used in Direct Speech. It is also used in Reported Speech when it is not followed by the person the words were spoken... [read more]
Earth sheltering is the architectural practice of using earth against building walls for external thermal mass, to reduce heat loss, and to easily maintain a steady indoor air temperature. Earth sheltering is popular in modern times among advocates of passive solar and sustainable architecture, but has been around for nearly as long as humans have been constructing their own shelter. The benefits of earth sheltering are numerous. They include: taking advantage of the earth as a thermal mass,...
